Fix markup on form-pay.php

Payment method fields were not displaying on the "Checkout -> Pay" page because `checkout.js` was using a `input#payment_method_{name}` selector, which works fine with the payment method markup used in the `review-order.php` template, but not with the markup used in the `form-pay.php` template.

This moves the `#payment_method_{name}` attribute from the `<li/>` elements to the `<input/>` element so that `checkout.js` correctly displays payment method fields.