PEP 828: Update reference implementation link (#4858)

Author: ZeroIntensity

peps/pep-0828.rst

@@ -329,8 +329,8 @@ For example:
 Reference Implementation
 ========================
 
-A reference implementation of this PEP can be found
-`here <https://github.com/python/cpython/compare/main...zerointensity:cpython:async-yield-from>`__.
+A reference implementation of this PEP can be found at
+`python/cpython#145716 <https://github.com/python/cpython/pull/145716>`__.
 
 
 Rejected Ideas