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

[puf]

snippets-web/snippets/firestore-next/test-firestore/get_custom_object.js

Line 16 in 1abb6ce

console.log(city.toString());

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.