Fix field names in bank account form

I don't have time to dig around and figure out if there is Python somewhere that needs changing as well, but I though I'd point this out.
Nor is country (it is for the [credit card JS](https://github.com/gratipay/gratipay.com/blob/1864e6771c8e139149ce0a7dfa3a7db388ecef32/js/gratipay/payments.js#L400), but not for the bank account JS).