Fix small documentation inconsistency

Hey, super small change here. I just noticed the comment in RSVP documentation is slightly wrong.

`reason.message` comes from https://github.com/amiel/ember.js/blob/46fa5bb896548d88a3e056d2903c0b5b0cffa6cf/packages/rsvp/lib/main.js#L1790

Thanks!

ref emberjs/ember.js#4922