clarify lock protection cookbook to specify fieldname

Resources/doc/cookbook/recipe_lock_protection.rst

@@ -30,7 +30,7 @@ You can enable it in your ``sonata_admin`` configuration :
             options:
                 lock_protection: true
                 
-You must also configure each entity that you want to support by adding a ``version`` field.
+You must also configure each entity that you want to support by adding a field called ``$version`` on which the Doctrine ``Version`` feature is activated.
 
 .. code-block:: php