ipa_client_automount.py: fix typo (idmap.conf => idmapd.conf)
    
    660c49 introduced --idmap-domain which sets the Domain option in
    idmapd.conf. However the help message for that knob mentioned
    idmap.conf which is wrong. Fix that.
    Reported by Marc Muehlfeld <mmuehlfe@redhat.com>.
    
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Michal Polovka <mpolovka@redhat.com>