Doc: update live-migration cmd

The deprecated `--live` option of the `server migrate` command
was removed I37ef09eca0db9286544a4b0bb33f845311baa9b2

Update docs with new arguments.

Change-Id: Id7a9a7509ca5e7811b6d3ce060390ea23c93d4ce

Author: Arefiev Anton

doc/source/admin/live-migration-usage.rst

@@ -102,7 +102,7 @@ Manual selection of the destination host
 
    .. code-block:: console
 
-      $ openstack server migrate d1df1b5a-70c4-4fed-98b7-423362f2c47c --live HostC
+      $ openstack server migrate d1df1b5a-70c4-4fed-98b7-423362f2c47c --live-migration --host HostC
 
 #. Confirm that the instance has been migrated successfully: