Tier 1 link in README doesn't go anywhere

https://github.com/japaric/rust-cross#i-want-to-build-binaries-for-linux-mac-and-windows-how-do-i-cross-compile-from-linux-to-mac

Link tries to go to https://doc.rust-lang.org/book/getting-started.html#tier-1, but even after clicking through the outdated book warning, there's no mention of "tier" on the page