fix api bug (#909)

tools/export_serving_model.py

@@ -50,7 +50,11 @@ def save_serving_model(FLAGS, exe, feed_vars, test_fetches, infer_prog):
     import paddle_serving_client.io as serving_io
     serving_client = os.path.join(save_dir, 'serving_client')
     serving_server = os.path.join(save_dir, 'serving_server')
-    serving_io.save_model(serving_client, serving_server, feed_dict, fetch_dict,
+    serving_io.save_model(
+        client_config_folder=serving_client,
+        server_model_folder=serving_server,
+        feed_dict,
+        fetch_dict,
         infer_prog)