Update link to nova config reference

For devstack master branch we should point to the latest version and not
to some possibly outdated older branch.

Change-Id: I4af3aef90a2c295df3de4a5b49d127e85ab517ac

Author: jharbott

doc/source/guides/nova.rst

@@ -66,5 +66,5 @@ These config options are defined in `nova.conf.serial_console
 <https://github.com/openstack/nova/blob/master/nova/conf/serial_console.py>`_.
 
 For more information on OpenStack configuration see the `OpenStack
-Configuration Reference
-<https://docs.openstack.org/ocata/config-reference/compute.html>`_
+Compute Service Configuration Reference
+<https://docs.openstack.org/nova/latest/admin/configuration/index.html>`_