website: fix use case always highlighting

Author: mitchellh

website/source/layouts/intro.erb

@@ -6,7 +6,7 @@
 				<a href="/intro/index.html">What is Terraform?</a>
                 </li>
 
-				<li<%= sidebar_current("what") %>>
+				<li<%= sidebar_current("use-cases") %>>
 				<a href="/intro/index.html">Use Cases</a>
 				</li>