message: Fix install link.
  The new website broke the old links. They are setting up redirects (https://github.com/rust-lang/www.rust-lang.org/pull/597), but I figured may as well use the new location to avoid the redirect.
diff: @@ -33,5 +33,5 @@ Alternatively, you can [build Cargo from source][compiling-from-source].
  [rust]: https://www.rust-lang.org/
  [rustup-init.exe]: https://win.rustup.rs/
  -[install-rust]: https://www.rust-lang.org/install.html
  +[install-rust]: https://www.rust-lang.org/tools/install
  [compiling-from-source]: https://github.com/rust-lang/cargo#compiling-from-source
  