Update MaybeNone explanation location (#14630)

Author: Avasam

stdlib/_typeshed/__init__.pyi

@@ -54,7 +54,8 @@ Unused: TypeAlias = object  # stable
 
 # Marker for return types that include None, but where forcing the user to
 # check for None can be detrimental. Sometimes called "the Any trick". See
-# CONTRIBUTING.md for more information.
+# https://typing.python.org/en/latest/guides/writing_stubs.html#the-any-trick
+# for more information.
 MaybeNone: TypeAlias = Any  # stable
 
 # Used to mark arguments that default to a sentinel value. This prevents