Update models.md

[shobu13]

reference: model.model (1251)

        if not getattr(new_class._meta, "model_key_prefix", None):
            # Don't look at the base class for this.
            new_class._meta.model_key_prefix = (
                f"{new_class.__module__}.{new_class.__name__}"
            )

so the property default to the module name + class name instead of just "", current documentation may missleadv :)