Fix labelPath in VariantChoiceList 

Otherwise you get the exception

> An exception has been thrown during the rendering of a template ("Neither the property "title" nor one of the methods "getTitle()", "isTitle()", "hasTitle()", "__get()" or "__call()" exist and have public access in class "Sylius\Bundle\CoreBundle\Model\Variant".") in SyliusWebBundle:Frontend/Product:show.html.twig at line 3.