book: nightly not updated for rustup

[steveklabnik]

https://doc.rust-lang.org/stable/book/nightly-rust.html still shows rustup.sh

I am happy to mentor anyone who wants to tackle this.

[XAMPPRocky]

Oh, sorry when the person on IRC brought that up, I already went and made a PR for it. #39725

[Mark-Simulacrum]

This can be closed; PR was merged and now just needs to roll through. Unless we consider it important enough for a backport?

[steveklabnik]

Nope, thanks!