fix documentation updateAttributeGroup() (OpenMage#1593)

Author: petrsvamberg

app/code/core/Mage/Eav/Model/Entity/Setup.php

@@ -489,7 +489,7 @@ public function addAttributeGroup($entityTypeId, $setId, $name, $sortOrder = nul
      * @param mixed $entityTypeId
      * @param mixed $setId
      * @param mixed $id
-     * @param string $field
+     * @param array|string $field
      * @param mixed $value
      * @return $this
      */