[Validator] Add a missing namespace and use

Author: wkania

validation/custom_constraint.rst

@@ -285,6 +285,11 @@ not to the property:
 
     .. code-block:: php-attributes
 
+        // src/Entity/AcmeEntity.php
+        namespace App\Entity;
+
+        use App\Validator as AcmeAssert;
+
         #[AcmeAssert\ProtocolClass]
         class AcmeEntity
         {