message: [openwrt] update doc to use `service`
diff: @@ -63,7 +63,7 @@ Start otbr-agent manually:
  Edit the service file `/etc/init.d/otbr-agent` if RCP device is not `/dev/ttyUSB0` and then start with:
  ```bash
  -start_service otbr-agent
  +service otbr-agent start
  ```
  ### Test
  