Missing single quote on the Buttons element usage example

API page: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21...

Problem/Motivation

There is a missing quote after #value on the Usage example:

<?php
$form['actions']['preview'] = array(
  '#type' => 'button',
  '#value => $this->t('Preview'),
);
?>

Proposed resolution

Should be:

<?php
$form['actions']['preview'] = array(
  '#type'  => 'button',
  '#value' => $this->t('Preview'),
);
?>