docs: fix broken link in DEVELOPER.md (angular#37212)

The TOC at the top refers to an anchor that does not exist.

This change adds the missing anchor while preserving the old one in case someone depends on it.

PR Close angular#37212

docs/DEVELOPER.md

@@ -89,6 +89,7 @@ benefit from Bazel's capability to do incremental builds.
 All the tests are executed on our Continuous Integration infrastructure. PRs can only be
 merged if the code is formatted properly and all tests are passing.
 
+<a name="formatting-your-source-code">
 <a name="clang-format"></a>
 ## Formatting your source code