message: [agent] print help message for backbone interface
diff: @@ -189,7 +189,7 @@ static int Mainloop(otbr::AgentInstance &aInstance, const char *aInterfaceName)
  static void PrintHelp(const char *aProgramName)
  {
  - fprintf(stderr, "Usage: %s [-I interfaceName] [-d DEBUG_LEVEL] [-v] RADIO_URL\n", aProgramName);
  + fprintf(stderr, "Usage: %s [-I interfaceName] [-B backboneIfName] [-d DEBUG_LEVEL] [-v] RADIO_URL\n", aProgramName);
  fprintf(stderr, "%s", otSysGetRadioUrlHelpString());
  }
  