[Form]: Update entity.rst: Adding __toString() hint

Page: https://symfony.com/doc/6.4/reference/forms/types/entity.html#using-a-custom-query-for-the-entities

Author: ThomasLandauer

reference/forms/types/entity.rst

@@ -64,7 +64,7 @@ returning a ``QueryBuilder`` object or ``null`` to load all entities)::
             return $er->createQueryBuilder('u')
                 ->orderBy('u.username', 'ASC');
         },
-        'choice_label' => 'username',
+        'choice_label' => 'username', // or create `__toString()` in the entity
     ]);
 
 .. note::