code: func (s *ActivityService) MarkRepositoryNotificationsRead(ctx context.Context, owner, repo string, lastRead time.Time) (*Response, error) {
  	opts := &markReadOptions{
  		LastReadAt: lastRead,
  	}
  	u := fmt.Sprintf("repos/%v/%v/notifications", owner, repo)
  	req, err := s.client.NewRequest("PUT", u, opts)
  	if err != nil {
  		return nil, err
  	}
  
  	return s.client.Do(ctx, req, nil)
  }
docs: // MarkRepositoryNotificationsRead marks all notifications up to lastRead in
  // the specified repository as read.
  //
  // GitHub API docs: https://developer.github.com/v3/activity/notifications/#mark-notifications-as-read-in-a-repository