message: fix SlackAdapter datepicker typo
diff: @@ -549,7 +549,7 @@ export class SlackAdapter extends BotAdapter {
  activity.text = event.actions[0].selected_channel;
  break
  case 'datepicker':
  - activity.text = event.action[0].selected_date;
  + activity.text = event.actions[0].selected_date;
  break
  default: activity.text = event.actions[0].type;
  }
  