add db_path param to node route_guide_server command

node/route_guide/README.md

@@ -354,7 +354,7 @@ $ npm install
 ```
 Run the server, which will listen on port 50051:
 ```shell
-$ node ./route_guide_server.js
+$ node ./route_guide_server.js --db_path=route_guide_db.json
 ```
 Run the client (in a different terminal):
 ```shell