docs: Update Freenode to OFTC

Change-Id: I3224ca9b96a181de8c0251d0bd589135e098d557

Author: priteau

doc/source/contributor/contributing.rst

@@ -11,7 +11,7 @@ with python-cloudkittyclient.
 
 Communication
 ~~~~~~~~~~~~~
-* IRC channel #cloudkitty at FreeNode
+* IRC channel #cloudkitty at `OFTC <http://oftc.net>`_
 * Mailing list (prefix subjects with ``[cloudkitty]`` for faster responses)
   http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-discuss