In the current book, recommend rustup.rs over rustup.sh

[carols10cents]

Specifically at https://doc.rust-lang.org/book/getting-started.html#installing-on-linux-or-mac, and recommend rustup.rs for Windows too, right?

We've switched the new book to do this already, but in the meantime, the old book should recommend the new recommended toolchain :)

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 a bit, and we'd need to then make sure to update the website in sync (see rust-lang/prev.rust-lang.org#478)

[est31]

Maybe the big red warning "this is beta software" should be removed from the https://rustup.rs website?

[frewsxcv]

This will be fixed by #38122.