ref fissix via intersphinx

Co-authored-by: Kyle Altendorf <[email protected]>

Author: graingert

docs/fixers.rst

@@ -232,7 +232,7 @@ version of ``six`` is installed.
 ``fissix`` fixers
 +++++++++++++++++
 
-Some `fixers from fissix <https://docs.python.org/3/library/2to3.html#fixers>`_
+Some :doc:`fixers from fissix <fissix:fixers>`
 in Python's standard library are run by default unmodified as their
 transformations are Python 2 compatible.