use a ref instead of hard-coded link in new tip

Use a ref role instead of a hard-coded link to let the link work no
matter where the documentation is being served from.

Author: dhellmann

docs/source/tips.rst

@@ -147,8 +147,8 @@ Clean up environments on exit
 
 Via `Michael <https://github.com/kojiromike>`__:
 
-When you use a temporary virtualenv via ``mktmpenv`` or if you have
-a `deactivate <http://virtualenvwrapper.readthedocs.org/en/latest/plugins.html?highlight=hooks#post-deactivate>`_ hook, you have to actually run
+When you use a temporary virtualenv via ``mktmpenv`` or if you have a
+:ref:`plugins-post_deactivate` hook, you have to actually run
 ``deactivate`` to clean up the temporary environment or run the hook,
 respectively. It's easy to forget and just exit the shell. Put the
 following in ``~/.bash_logout`` (or your shell's equivalent file) to