Add a link to _typeshed readme

Author: Drino

docs/source/libraries.rst

@@ -503,7 +503,7 @@ different manner, so the most portable and easy way to create a shortcut
 is to define a callable ``Protocol`` as described in `PEP 
 544 <https://peps.python.org/pep-0544/#callback-protocols>`_.
 
-There is already a ``Protocol`` called ``IdentityFunction`` defined in ``_typeshed``:
+There is already a ``Protocol`` called ``IdentityFunction`` defined in `_typeshed <https://github.com/python/typeshed/blob/master/stdlib/_typeshed/README.md>`_:
 
 .. code:: python