Fix lies in the concurrency guide.

This cleans up blatant lies in the concurrency guide, and modernizes it
a bit. There's a lot more to do, but until I get to it, let's make it a
little bit better.
Perhaps:

> In particular, the type system guarantees that tasks cannot induce a data race from shared mutable state.

It's very important to point out that rust *does* have shared mutable state, it just doesn't have it will all the "oh dear please be careful" strings attached.