Add second level of headings for the section "Technical writing".

README.md

@@ -9,28 +9,35 @@ Technical writing is an essential skill of conveying complex technical informati
 > [!TIP] 
 > A few resources are unaccessible in some countries, such as China. If an alternative is available in a specific country, it is listed as a sublist item of the unaccessible one followed by a mark of `Country Name` in a pair of parentheses.
 
-Want to suggest a resource? Check out the [contribution guidelines](https://github.com/wongyah/awesome-technical-writing-learning/blob/main/contributing.md)! 👐
+Want to suggest a resource? Check out the [Contribution Guidelines](https://github.com/wongyah/awesome-technical-writing-learning/blob/main/contributing.md)! 👐
 
 ---
 
 <!-- omit from toc -->
 ## Contents
 
-- [Technical Writing](#technical-writing)
-- [Technical Illustrations](#technical-illustrations)
-- [Authoring Formats](#authoring-formats)
+- [Technical writing](#technical-writing)
+  - [Courses for beginners](#courses-for-beginners)
+  - [Books](#books)
+  - [Editorial style guides for technical writing](#editorial-style-guides-for-technical-writing)
+  - [General purpose editorial style guides](#general-purpose-editorial-style-guides)
+  - [Dictionaries](#dictionaries)
+- [Technical illustrations](#technical-illustrations)
+- [Authoring formats](#authoring-formats)
   - [Markdown](#markdown)
   - [reStructuredText](#restructuredtext)
   - [DITA](#dita)
   - [S1000D](#s1000d)
-- [Text Automatic Processing](#text-automatic-processing)
-  - [Regular Expression](#regular-expression)
+- [Text automatic processing](#text-automatic-processing)
+  - [Regular expression](#regular-expression)
   - [XSL](#xsl)
   - [PowerShell](#powershell)
 
 ---
 
-## Technical Writing
+## Technical writing
+
+### Courses for beginners
 
 If you are completely new to technical writing, the following **🔔 courses** are the best start points for you:
 
@@ -39,6 +46,8 @@ If you are completely new to technical writing, the following **🔔 courses** a
 - [GitLab Technical Writing Fundamentals Course](https://handbook.gitlab.com/handbook/product/ux/technical-writing/fundamentals/) - A four-video course teaching how to write and edit GitLab documentation.
 - [Coursera: Introduction to Technical Writing](https://www.coursera.org/learn/technical-writing-introduction) - A systematic introduction to technical writing, including the features, workflow, software and tools, history and futures, etc.
 
+### Books
+
 Now, you have known a bit of technical writing 👏! Not enough for you? Let's go further 🤠!
 
 To learn a new skill or a new subject systematically, there is nothing better than **📚 academic textbooks and ebooks** self-published by experienced professionals at work. Here you are, enjoy 😄!
@@ -49,6 +58,8 @@ To learn a new skill or a new subject systematically, there is nothing better th
 - [Developing Quality Technical Information: A Handbook for Writers and Editors (IBM Press)](https://www.amazon.com/Developing-Quality-Technical-Information-Handbook/dp/0133118975) - A must-read book of technical writers for all time, even though it is not a free resource.
 - [Open Technical Communication](https://alg.manifoldapp.org/projects/open-technical-communication) - An open textbook authored by five professionals at Kennesaw State University, which has been revised and updated to 4th edition.
 
+### Editorial style guides for technical writing
+
 Learning by doing, rather than learning by reading. It's time to write or imporve your own technical documentation using the writing skills you have studied!
 
 But wait a minute, you still need to adopt an **📙 editorial style guide for technical writing** at first. If you never know or forget what editorial sytle guides are or why you need one, go back to the [Google Technical Writing Courses for Engineers](https://developers.google.cn/tech-writing/resources) for a refresher.
@@ -63,18 +74,22 @@ But wait a minute, you still need to adopt an **📙 editorial style guide for t
 - [SUSE Documentation Style Guide](https://documentation.suse.com/en-us/style/current/single-html/docu_styleguide/#) - A style guide for software documentations, which emphasizes a professional tone and comprehensive markup reference for DocBook.
 - [Apple Style Guide](https://support.apple.com/zh-cn/guide/applestyleguide/welcome/web) - A style guide for Apple materials, nothing special.
 
-In the real world, anything is possible. Take a suitable **📙 general propose editorial style guide** at hand, in case you step into an unknown situation someday.
+### General purpose editorial style guides
+
+In the real world, anything is possible. Take a suitable **📙 general purpose editorial style guide** at hand, in case you step into an unknown situation someday.
 
 - [Chicago Manual of Style](https://chicagomanualofstyle.org/home.html) - A widely used style guide over 1000 pages, which was published continuously since 1906 and now is in its 17th edition (subscription required).
 - [University of Oxford Style Guide](https://www.ox.ac.uk/sites/files/oxford/University%20of%20Oxford%20Style%20Guide%20%28updated%20Hilary%20term%202016%29.pdf) - A 30-page sytle guide to writing and formatting documents consistently.
 - [University of Oxford Style Guide Quick Reference](https://www.ox.ac.uk/sites/files/oxford/Style%20Guide%20quick%20reference%20A-Z.pdf) - A one-page quick reference guide with the most commonly requested information.
 
+### Dictionaries
+
 And you know, select a suitable **📘 dictionary** too!
 
 - [American Heritage Dictionary](https://ahdictionary.com/)
 - [Merriam-Webster Dictionary](https://www.merriam-webster.com/)
 
-## Technical Illustrations
+## Technical illustrations
 
 A picture is worth a thousand words.
 
@@ -90,7 +105,7 @@ Technical illustrators used to use software like Adobe Illustrator or CAD to cre
 - [XVL Studio 3D CAD Corel Edition](https://www.coreldraw.com/en/product/technical-suite/xvl-studio/) - A commercial software for technical illustrations developed by CorelDRAW.
 - [XVL Technical Illustration Suite](https://www.lattice3d.com/products/xvl-technical-illustration-suite) - A suite of commercial softwares for technical illustrations developed by Lattice Technology.
 
-## Authoring Formats
+## Authoring formats
 
 Going with the trend of Docs as Code, more and more organizations and their technical writers have ditched word processors and switched to markup languages.
 
@@ -202,11 +217,11 @@ Here are the **📁 official specifications and implementations in specific indu
 - [S1000D DIG Business Rules](https://www.gerkemulder.nl/DIG_Business_Rules_issue_1.0_170619.pdf) - An implementation of S1000D specification in defense industries.
 - [The Shipdex Protocol: Issue 2.3](http://www.shipdex.org/shipdex-viewer-free.asp) - An implementation of S1000D specification in shipping industry.
 
-## Text Automatic Processing
+## Text automatic processing
 
 Equipping yourself with some programming skills will enable you to complete some really cool jobs, such as automating workflow and batch modifying.
 
-### Regular Expression
+### Regular expression
 
 A regular expression (regex) is a pattern that the regex engine can use to find or match substrings. Regex is extremely useful in finding and replacing texts or strings in text files.