get_custom_object and other tests/snippets should not rely on `toString` to verify custom object types

[puf]

https://github.com/firebase/snippets-web/blob/1abb6ce1a784ae5552946dff5f1f5aab7dcbda30/snippets/firestore-next/test-firestore/get_custom_object.js#L16

The toString() method exists on each Object already, so it is not a great example to highlight that we get a custom object type back when using withConverter. I recommend using name, state, and/or country instead.