'Shortcut icon settings' not using proper FAPI value

If a module adds:

$form['favicon']['#collapsible'] = true;
$form['favicon']['#collapsed'] = true;

The favicon fieldset won’t collapse properly because the fieldset’s description is placed as a separate, un-wrapped 'markup' instead of a '#description' of the fieldset. This patch only changes $form['favicon']['text'] to $form['favicon']['#description'].