Updated an old link int he contributing document.

CONTRIBUTING.rst

@@ -49,7 +49,7 @@ To set up `apex` for local development:
 
    Now you can make your changes locally.
 
-4. When you're done making changes, run all the checks, doc builder and spell checker with `tox <http://tox.readthedocs.org/en/latest/install.html>`_ one command::
+4. When you're done making changes, run all the checks, doc builder and spell checker with `tox <http://tox.readthedocs.io/en/latest/install.html>`_ one command::
 
     tox