[docs][clang] Fix a broken link on the APINotes doc

[JohnTitor]

Summary

This patch replaces a link with a GitHub one.

Fixes llvm#55748

[compnerd]

It seems that other docs are following the GitHub URLs as well. I wish we could do the relative document link instead of embedding the URL, but if that is not possible, then this makes sense to do.

[JohnTitor]

It seems that other docs are following the GitHub URLs as well. I wish we could do the relative document link instead of embedding the URL, but if that is not possible, then this makes sense to do.

I couldn't find the URL serving that file and I'm not familiar with doing that. Let's use a GitHub URL for now.

clang/docs/APINotes.rst

 
 **Better solution:** Provide a "sidecar" file with the information you want to
 add, and have that automatically get picked up by the module-building logic in
 the compiler.
 
 That's API notes.
 
 API notes use a YAML-based file format. YAML is a format best explained by
-example, so here is a `small example`__ from the compiler test suite of API
+example, so here is a `small example
+<https://github.com/llvm/llvm-project/blob/main/clang/test/APINotes/Inputs/Frameworks/SomeKit.framework/Headers/SomeKit.apinotes>`_
+from the compiler test suite of API
 notes for a hypothetical "SomeKit" framework.
 
-__ test/APINotes/Inputs/Frameworks/SomeKit.framework/Headers/SomeKit.apinotes
-
 
 Usage
 =====
 
 API notes files are found relative to the module map that defines a module,
 under the name "SomeKit.apinotes" for a module named "SomeKit". Additionally, a
 file named "SomeKit_private.apinotes" will also be picked up to go with a
 private module map. For bare modules these two files will be in the same