Fix readme: Http header syntax highlighting

[NickNiebling]

Rationale for this PR

This PR is a minor semantic fix to readme files of GitHub and NuGet, that adds syntax highlighting for http header code block.

Org (using plaintext):

 cache-control: max-age=31536000,private 
 content-security-policy: script-src 'self';object-src 'self';block-all-mixed-content;upgrade-insecure-requests; 
 cross-origin-resource-policy: same-origin 
 referrer-policy: no-referrer 
 strict-transport-security: max-age=31536000;includeSubDomains 
 x-content-type-options: nosniff 
 x-frame-options: DENY 
 x-permitted-cross-domain-policies: none; 
 x-xss-protection: 0 

Suggested (using http - and removing leading whitespace):

cache-control: max-age=31536000,private 
content-security-policy: script-src 'self';object-src 'self';block-all-mixed-content;upgrade-insecure-requests; 
cross-origin-resource-policy: same-origin 
referrer-policy: no-referrer 
strict-transport-security: max-age=31536000;includeSubDomains 
x-content-type-options: nosniff 
x-frame-options: DENY 
x-permitted-cross-domain-policies: none; 
x-xss-protection: 0 

PR Checklist
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Essential
These items are essential and must be completed for each commit. If they are not completed, the PR may not be accepted.

❓ I have added tests to the OwaspHeaders.Core.Tests project
❓ I have run the dotnet-format command and fixed any .editorconfig issues
❓ I have ensured that the code coverage has not dropped below 65%
❓ I have increased the version number in OwaspHeaders.Core.csproj (only relevant for code changes)

Optional
❓ I have documented the new feature in the docs directory

Any Other Information

No other information

[GaProgMan]

This is fantastic, thank you for this PR

[GaProgMan]

One thing to note is that the PR will start a build, which will fail, when I merge it. In this instance, I'm OK with the build failure, as I have a set of code changes coming up, and will fix the build with that PR (#164 )

[NickNiebling]

One thing to note is that the PR will start a build, which will fail, when I merge it. In this instance, I'm OK with the build failure, as I have a set of code changes coming up, and will fix the build with that PR (#164 )

I'm in sorry about that. I honestly didn't touch any code and expected to edit readme on a stable build 🙈 at least I was honest about how little I verified the commit wasn't breaking 🤣

Happy new year - and thanks for the modern .NET show 🙌