broken link: ratpack.io streams

[raboof]

stream/reactive-streams-interop.html has a link to https://ratpack.io/manual/current/streams.html which is a 404

[Philippus]

Maybe intermittent, I saw it work a week ago. But we could change the link to https://github.com/ratpack/ratpack.

[pjfanning]

Makes sense to change.