Fix docs: Cookbook form_ajax_refs definition

[j123b567]

I really don't understand it deeply but the original code example for form_ajax_refs from the cookbook does not work.
It is not documented anywhere why, I just found the answer in tests/test_ajax.py.

• There is no element child in the Parent class but there is a children relationship
• "order_by": ("id",) just does not work (sqlalchemy.exc.ArgumentError: ORDER BY expression expected, got ('id',).) but "order_by": "id" works

It would be also fine to add __str__ method to the example implementation of the class Child to be more clear about what is used in the list.

[aminalaee]

Thank you for the contribution 👍