Fix crossbeam-channel link & update example

[milkey-mouse]

This is a very trivial change, but it saves a couple clicks for every visitor. crossbeam-channel is now part of the main crossbeam repository, so link to its (crossbeam-channel's) README instead of crossbeam's.

[ghost]

While we're at it, could you perhaps update the first example in the readme (the one with signals)? It was written in the v0.2 era when send() wasn't returning anything. Today, send() returns a Result.

So the suggestion is to change s.send(signal); to:

if s.send(signal).is_err() {
    break;
}

[BurntSushi]

Looks good with @stjepang's suggestion!

[milkey-mouse]

I added that change in a separate commit.

[ghost]

Thanks!

[BurntSushi]

Thanks!