migration: fix one typo in comment of function migration_total_bytes()

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190912024957.11780-1-richardw.yang@linux.intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>