Fix markup on form-pay.php

Payment method fields were not displaying on the "Checkout -> Pay" page in WC 2.1 because `checkout.js` uses a `input#payment_method_{name}` selector, which is 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 a payment method's fields.