Recommend rustup.rs over rustup.sh

[carols10cents]

Specifically at https://www.rust-lang.org/en-US/downloads.html, and recommend rustup.rs for Windows too, right?

We've switched the new book to do this already :)

If it's too soon to do this because rustup.rs isn't stable enough yet, consider this a reminder issue to make this change once it is :)

[steveklabnik]

@brson wants to wait another release cycle or two. And we'd need to update the current book as well.

[brson]

I'm very reluctant to do so without resolving this: rust-lang/rustup#524

[alexcrichton]

Done!

🎆