Update Guide snippet error

[bwignall]

The output on the Guide/Concurrency page currently shows

Stream(1,2,3).merge(Stream.eval(IO { Thread.sleep(200); 4 })).compile.toVector.unsafeRunSync()
// error: Could not find an implicit IORuntime.
// 
// Instead of calling unsafe methods directly, consider using cats.effect.IOApp, which
// runs your IO. If integrating with non-functional code or experimenting in a REPL / Worksheet,
// add the following import:

and the text Oops, we need a cats.effect.ContextShift[IO] in implicit scope..

The text was last updated August 2018, before cats-effect 3, which changed ContextShift. I updated it to refer to IORuntime.

[armanbilge]

Ugh, this guide is old. There are many things wrong with it. This Thread.sleep should be rewritten as IO.sleep(...).as(4) or something.

[armanbilge]

Merging b/c this fix is good and standalone (thanks!). We should cleanup some other things as well.