Fix format in live-migration-usage.rst
Fix format in live-migration-usage.rst

It is :command:`some_command` instead of :command:``some_command``.

Change-Id: I8300c55a247d30cb9db93e5025d79d0fdc24cf5e

phenoxim/nova,mikalstill/nova,openstack/nova,gooddata/openstack-nova,phenoxim/nova,mahak/nova,klmitch/nova,klmitch/nova,gooddata/openstack-nova,rahulunair/nova,rahulunair/nova,gooddata/openstack-nova,rahulunair/nova,openstack/nova,klmitch/nova,mahak/nova,mahak/nova,gooddata/openstack-nova,mikalstill/nova,openstack/nova,mikalstill/nova,klmitch/nova