Fixed typo in example code for Custom Validators

I could be wrong, but I believe the following code is required to be able to add a validator. The problem I was having was with the example saying `$model->validator` rather than `$model->_validator` (and causing Paris to throw an error because the property does not exist).

I also found that I needed to call `$model->setupValidationEngine()` because otherwise the engine is not setup until `$model->validateField()` is called.