Update NodeInterface.php fix return type for getCustomerGroups function

Add missing @return type to recently added getCustomerGroups function.

Api/Data/NodeInterface.php

@@ -302,7 +302,7 @@ public function setSelectedItemId($selectedItemId);
 
     /**
      * Get customer groups
-     *
+     * @return mixed[]
      */
     public function getCustomerGroups();