Use "registrable domain" instead of "eTLD+1"

[zcorpan]

https://w3c.github.io/payment-request/#canmakepayment-protections

For rate-limiting the user agent might look at repeated calls from:

• the same effective top-level domain plus one (eTLD+1).
• the top-level browsing context. Alternatively, the user agent may block access to the API entirely for origins know to be bad actors.
• the origin of an iframe or popup window.
  These rate-limiting techniques intend to increase the cost associated with repeated calls, whether it is the cost of managing multiple eTLDs or the user experience friction of opening multiple windows (tabs or pop-ups).

eTLD+1 is now defined as "registrable domain" in the URL spec:

https://url.spec.whatwg.org/#host-registrable-domain

Also, some other terms in the quoted text could link to the defined term (top-level browsing context, origin).