hub.challenge is not part of the subscriber's rest hooks request to unsubscribe

[isaacvetter]

We mistakenly documented hub.challenge as part of a subscriber's request to unsubscribe. This is an error, the function of the Hub-generated hub.challenge is to be passed to the subscriber during intent verification and merely echoed by the subscriber as a single confirmation of intent.

This should be modified from the Unsubscribe section:

Only unsubscribes for hub.channel.type=webhook SHALL include the hub.callback, hub.secret, and hub.challenge.

hub.challenge | Conditional | string | Required when hub.channel.type=webhook. SHALL not be present when hub.channel.type=websocket. A Hub-generated, random string communicated during Intent Verification.

hub.channel.type=webhook&hub.callback=https%3A%2F%2Fapp.example.com%2Fsession%2Fcallback%2Fv7tfwuk17a&hub.mode=unsubscribe&hub.topic=fdb2f928-5546-4f52-87a0-0648e9ded065&hub.secret=shhh-this-is-a-secret&hub.events=patient-open,patient-close&hub.challenge=meu3we944ix80ox