Fix for server.events.on misled by release notes typo

[jrgm]

Re: #2542 and #2543.

The release notes said channel (singular), but the api docs say channels (plural):

hapijs/hapi#3658
https://github.com/hapijs/hapi/blob/master/API.md#-servereventsoncriteria-listener

If I use channels instead, it works locally. In stage with channel, I get "channel" is not allowed and startup up fails with SENTRY_DSN set in env.