Don't offline after propose-requirements-updates

* modules/openstack_project/files/zuul/layout.yaml: The
propose-requirements-updates job name is plural, unlike other
propose-.*-update jobs, so add an optional s? at the end of the
pattern. Without this, proposal.slave.openstack.org gets set offline
every time that job is completed.

Change-Id: I0f7d8366a094666945af7387dba3623b393e9659