Update invitation link (#260)

Author: alalazo

common/setup.rst

@@ -14,4 +14,4 @@
 
    See the :ref:`basics-tutorial` for full details on setup. For more
    help, join us in the ``#tutorial`` channel on Slack -- get an
-   invitation at `spackpm.herokuapp.com <https://spackpm.herokuapp.com>`_
+   invitation at `slack.spack.io <https://slack.spack.io/>`_