Fix systemctl option: removes --unit option in doc
Fix systemctl option: removes --unit option in doc

--unit option is for journalctl, not systemctl.
Just executing systemctl without "--unit=" works.

Change-Id: I9752561332e62ec6327b17b12d2d868892718041

group-policy/devstack,noironetworks/devstack,nawawi/openstack,richliu/devstack,richliu/devstack,dtroyer/devstack,dtroyer/devstack,nawawi/openstack,group-policy/devstack,noironetworks/devstack,openstack-dev/devstack,openstack-dev/devstack