Update broken link
Update broken link

A bunch of files in the wunderboss project moved in this commit:
https://github.com/projectodd/wunderboss/commit/fb0ed61a65c04f731f667553397d49d794f7e41e,
breaking this link.

Note: the "default versions" link in docs/guides/messaging.md may
need to be updated as well someday, but since it's pointing to a branch,
it's fine for now.

kbaribeau/immutant,immutant/immutant,immutant/immutant,kbaribeau/immutant,immutant/immutant,immutant/immutant,kbaribeau/immutant