Fix SPEC link

Author: leahneukirchen

index.html

@@ -144,7 +144,7 @@ <h2>News</h2>
 
 <h2>Documentation</h2>
 
-<p>Read <a href="http://rack.rubyforge.org/doc/files/SPEC.html">The Rack interface specification</a>
+<p>Read <a href="http://rack.rubyforge.org/doc/SPEC.html">The Rack interface specification</a>
 and <a href="http://rack.rubyforge.org/doc/">other documentation</a>.</p>
 
 <h2>Supported Handlers and Adapters</h2>