Link "about rustup" to the rustup book

Author: kraai

www/index.html

@@ -186,7 +186,7 @@
    · 
   <a href="https://rust-lang.github.io/rustup-components-history/">component availability</a>
   &nbsp;&middot;&nbsp;
-  <a href="https://github.com/rust-lang/rustup">about rustup</a>
+  <a href="https://rust-lang.github.io/rustup/">about rustup</a>
 </p>
 
 <script src="rustup.js"></script>