replace on_failure with on-failure

Signed-off-by:  徐俊杰<roollingstone@gmail.com>
Signed-off-by: XuPaco <roollingstone@gmail.com>

docker/types/services.py

@@ -152,7 +152,7 @@ class UpdateConfig(dict):
 class RestartConditionTypesEnum(object):
     _values = (
         'none',
-        'on_failure',
+        'on-failure',
         'any',
     )
     NONE, ON_FAILURE, ANY = _values