Add missing URL link to Modernize docs.

Doc/howto/pyporting.rst

@@ -379,7 +379,7 @@ supported by Python 2. You should also update the classifiers in your
 .. _cheat sheet: http://python-future.org/compatible_idioms.html
 .. _coverage.py: https://pypi.python.org/pypi/coverage
 .. _Futurize: http://python-future.org/automatic_conversion.html
-.. _Modernize:
+.. _Modernize: http://python-modernize.readthedocs.org/en/latest/
 .. _Porting to Python 3: http://python3porting.com/
 .. _Pylint: https://pypi.python.org/pypi/pylint
 .. _Python 3 Q & A: http://ncoghlan-devs-python-notes.readthedocs.org/en/latest/python3/questions_and_answers.html