code: func (s *UsersService) Unfollow(ctx context.Context, user string) (*Response, error) {
  	u := fmt.Sprintf("user/following/%v", user)
  	req, err := s.client.NewRequest("DELETE", u, nil)
  	if err != nil {
  		return nil, err
  	}
  
  	return s.client.Do(ctx, req, nil)
  }
docs: // Unfollow will cause the authenticated user to unfollow the specified user.
  //
  // GitHub API docs: https://developer.github.com/v3/users/followers/#unfollow-a-user