[docs] fix incorrect rst markups
[docs] fix incorrect rst markups

The incorrect md-style markups break Sphinx builds,
which do not properly generate web page.

http://docs.openstack.org/developer/python-openstackclient/plugin-commands.html

Change-Id: I36ae2cf922836cff42653283c0a683359bd91344

dtroyer/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient,openstack/python-openstackclient