Fix docblock typehint on WebformHandlerInterface::setHandlerId()

Problem/Motivation

Static analysis tools disallow using WebformHandlerInterface::setHandlerId because it's typehinted incorrectly.

WebformHandlerBase:

  /**
   * The webform handler ID.
   *
   * @var string
   */
  protected $handler_id;

WebformHandlerInterface:

  /**
   * Returns the unique ID representing the webform handler.
   *
   * @return string
   *   The webform handler ID.
   */
  public function getHandlerId();

  /**
   * Sets the id for this webform handler.
   *
   * @param int $handler_id
   *   The handler_id for this webform handler.
   *
   * @return $this
   */
  public function setHandlerId($handler_id);