Link to the stable book by default

Author: alexcrichton

index.html

@@ -11,7 +11,7 @@
           prevents nearly all segfaults,
           and guarantees thread safety.
           <br/>
-          <a href="http://doc.rust-lang.org/nightly/book/README.html">Show me!</a>
+          <a href="http://doc.rust-lang.org/book/README.html">Show me!</a>
         </p>
       </div>
       <div class="col-md-4 install-box">