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
(cherry picked from commit 1d81fc891e1e7e56123a23609c00b55385b4e983)

vmturbo/nova,vmturbo/nova,vmturbo/nova,vmturbo/nova