PEP 526: Fix broken link to implementation (#4031)

Update pep-0526.rst

Author: ilevkivskyi

peps/pep-0526.rst

@@ -681,8 +681,8 @@ This PEP is fully backwards compatible.
 Implementation
 ==============
 
-An implementation for Python 3.6 is found on GitHub repo at
-https://github.com/ilevkivskyi/cpython/tree/pep-526
+An implementation for Python 3.6 can be found
+`on GitHub <https://github.com/python/cpython/commit/f8cb8a16a3>`_.
 
 
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