Fix example doc of named_model

[toshitanian]

fixed the usage example of named_model since it is incorrect comparing to the implementation.

• doc: https://pytest-factoryboy.readthedocs.io/en/stable/index.html#generic-container-classes-as-models
• implementation: https://github.com/pytest-dev/pytest-factoryboy/blob/ca5bb0971e7fa393ad4e999560d2ca3da046c85e/pytest_factoryboy/fixture.py#L73C1-L75C40

[youtux]

Thank you!

[toshitanian]

@youtux Hi! Thank you so much for your review. Is there something I should do on my side to get this PR merged?

[youtux]

Hi, I manually merged it, thank you for your contribution