Fix doc formating
Fix doc formating

Fix formating issue in the doc/source/plugin.rst

Trivial fix.

Change-Id: Ifc9a43aebdab0c15c83485cf0b6d4133d7194a9d

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