change contact link to GitHub issue tracker

The Jira issue tracker just tells folks to go to the GitHub issue tracker, so might as well direct them there from the start.

apx-contact.rst

@@ -1,10 +1,8 @@
 Contact Information
 ===================
 
-For support requests and bug reports, please submit a JIRA ticket in our
-`issue tracker <https://sft.its.cern.ch/jira/projects/CVM>`_. If you do not have
-a CERN account, you can register yourself for an
-`external account <https://account.cern.ch/account/Externals/RegisterAccount.aspx>`_.
+For support requests and bug reports, please submit a GitHub issue in our
+`issue tracker <https://github.com/cvmfs/cvmfs/issues>`_.
 
 Together with bug reports, please attach a "bugreport tarball", which is created
 with ``sudo cvmfs_config bugreport``.