message: mission: fix English spelling
diff: @@ -1252,8 +1252,8 @@ Mission::Result MissionImpl::build_mission_items(MAV_CMD command,
  }
  if (command == MAV_CMD_NAV_WAYPOINT) {
  - auto is_fly_thru = !(int(params[0]) > 0);
  - new_mission_item->set_fly_through(is_fly_thru);
  + auto is_fly_through = !(int(params[0]) > 0);
  + new_mission_item->set_fly_through(is_fly_through);
  }
  auto lat = params[4], lon = params[5];
  new_mission_item->set_position(lat, lon);
  