fix code style

python/paddle_serving_server/dag.py

@@ -13,7 +13,7 @@ class OpMaker(object):
             "general_text_response": "GeneralTextResponseOp",
             "general_single_kv": "GeneralSingleKVOp",
             "general_dist_kv_infer": "GeneralDistKVInferOp",
-            "general_dist_kv": "GeneralDistKVOp"
+            "general_dist_kv": "GeneralDistKVOp",
             "general_copy": "GeneralCopyOp",
             "general_detection":"GeneralDetectionOp",
         }