Fix typo in drupal_attach_tabledrag

The code in one of the code blocks doesn't parse right. There are missing single quotes.

@see the code block toward the end of https://api.drupal.org/api/drupal/core!includes!common.inc/function/drup...

foreach ($regions as $region) {
  drupal_attach_tabledrag('my-module-table', array(
    'action' => 'order',
    'relationship' => sibling',
    'group' => 'my-elements-weight',
    'subgroup' => my-elements-weight-' . $region,
  ));
}