Use bourne shell for rustup.sh

[zzmp]

Nothing's really broken, but rustup.sh does start with #!/bin/sh.

[alexcrichton]

PRs for the website should be made against https://github.com/wycats/cargo-website instead of this repo (these are just generated docs). I also believe that we explicitly use sh as it's likely to be practically everywhere and it's easier to type. Regardless, I would bring the issue up on cargo-website and likely on the official rust website as well as we'd like to keep the two in sync.