[libc][docs] Fix outdated code review section, as per llvm#91934 (llvm#92051)

As in the title, fixes llvm#91934

Author: cyuria

libc/src/math/docs/add_math_function.md

@@ -196,7 +196,8 @@ implementation (which is very often glibc).
 
 ## Code reviews
 
-We follow the code review process of LLVM with Phabricator:
+We use GitHub's inbuilt pull request system for code review:
 ```
-  https://llvm.org/docs/Phabricator.html
+  https://docs.github.com/articles/about-collaborative-development-models
+  https://docs.github.com/articles/about-pull-requests
 ```