CustomField example doesn't compile

[mvysny]

The example at https://vaadin.com/docs/latest/components/custom-field has couple of issues:

1. It doesn't compile: the PhoneField.generateModelValue() needs to return Phone instead of String
2. We should use TextField for code instead of Select, since Select accepts generic parameter but there's none in the example and thus a compiler warning is emitted; the Select dropdown is also not populated with country codes. TextField would probably be a simpler solution which would allow the reader to keep the focus on the CustomField example.