Fix hook_field_extra_fields_alter() example code

In core/modules/field/field.api.php there's a mistake in hook_field_extra_fields_alter() example. A key level is missed (['form'] or ['display']).

The function should be:

/**
 * Alter "pseudo-field" components on fieldable entities.
 *
 * @param $info
 *   The associative array of 'pseudo-field' components.
 *
 * @see hook_field_extra_fields()
 */
function hook_field_extra_fields_alter(&$info) {
  // Force node title to always be at the top of the list by default.
  foreach (node_type_get_types() as $bundle) {
    if (isset($info['node'][$bundle->type]['form']['title'])) {
      $info['node'][$bundle->type]['form']['title']['weight'] = -20;
    }
  }
}