code: func (cli *Client) ConfigUpdate(ctx context.Context, id string, version swarm.Version, config swarm.ConfigSpec) error {
  	if err := cli.NewVersionError("1.30", "config update"); err != nil {
  		return err
  	}
  	query := url.Values{}
  	query.Set("version", strconv.FormatUint(version.Index, 10))
  	resp, err := cli.post(ctx, "/configs/"+id+"/update", query, config, nil)
  	ensureReaderClosed(resp)
  	return err
  }
docs: // ConfigUpdate attempts to update a Config