Fixed link target for custom promise type docs

Ticket: ENT-11810
Changelog: None

getting-started/developing-modules.markdown

@@ -7,7 +7,7 @@ sorting: 50
 
 Modules, such as the one we've used for git promises, are easy to write.
 In this tutorial, we will focus on implementing a new promise type in Python, with the provided CFEngine library, since this is the easiest and recommended way.
-If you are interested in how modules are implemented, or how you could do it in another programming language, see the [complete documentation](/reference-promise-types-custom.html).
+If you are interested in how modules are implemented, or how you could do it in another programming language, see the [complete documentation][custom].
 
 In short, you need to implement 2 functions: `validate_promise()` and `evaluate_promise()`.
 _Validation_ should check that the correct attributes are used, and any other constraints you may want to enforce, to determine whether a promise is valid or invalid.