fix reference to renamed document

Author: xabbuh

book/forms.rst

@@ -638,7 +638,7 @@ large or whether you tried to submit text in a number field.
 .. seealso::
 
     To see how to use a service to resolve ``validation_groups`` dynamically
-    read the :doc:`/cookbook/validation/validation-group-service-resolver`
+    read the :doc:`/cookbook/validation/group_service_resolver`
     chapter in the cookbook.
 
 .. index::