[Validator] Add a missing namespace and use

Author: wkania

validation/custom_constraint.rst

@@ -228,6 +228,11 @@ not to the property:
 
     .. code-block:: php-annotations
 
+        // src/Entity/AcmeEntity.php
+        namespace App\Entity;
+
+        use App\Validator as AcmeAssert;
+        
         /**
          * @AcmeAssert\ProtocolClass
          */