[Docs] `withFieldGroup` corrections

[LeCarbonator]

Describe the bug

• The example lists onChangeListenTo in its field validators, even though that is not currently remapped to the form's field. While it is properly typed as only being string[], it would still be convenient to be able to listen to other group fields for validation. Either rewrite the snippet to not use it, or implement remapping for onChangeListenTo.
• The withFieldGroup section mentions that validators could be of any type. It's actually only form validators that could be of any type.

https://tanstack.com/form/latest/docs/framework/react/guides/form-composition#reusing-groups-of-fields-in-multiple-forms

TanStack Form version

v1.15.0