Update example showing ViewModel
Update example showing ViewModel

- Per the [mailing list](https://groups.google.com/a/zend.com/d/msgid/apigility-users/8be149da-1a6d-40a8-b1fa-97b4a3369a09%40zend.com?utm_medium=email&utm_source=footer),
  the example showing assignment of the entity to the view model was incorrect.
- Also provided a note on the `payload` view variable.

poisa/apigility-documentation,santiph/apigility-documentation,PowerKiKi/apigility-documentation