Correct the schema format

refer to https://github.com/openstack/watcher/blob/master/watcher/applier/actions/migration.py#L46

Change-Id: I2da8585ce978034fc9a79c2a0cf7bfb6eed487e2

watcher/applier/actions/volume_migration.py

@@ -45,7 +45,7 @@ class VolumeMigrate(base.BaseAction):
             'migration_type': str,  # choices -> "swap", "cold"
             'destination_node': str,
             'destination_type': str,
-        )}
+        })
 
     The `resource_id` is the UUID of cinder volume to migrate.
     The `destination_node` is the destination block storage pool name.