message: [json-rpc] fix invalid json example in method_get_metadata.md
  Closes:
diff: @@ -31,7 +31,7 @@ curl -X POST -H "Content-Type: application/json" --data '{"jsonrpc":"2.0","metho
  "result": {
  "timestamp": 1596680521771648,
  "version": 3253133,
  - "chain_id": 4,
  + "chain_id": 4
  }
  }
  ```
  