Improved hello world example

Calling `Rack::Server.start` seems more correct in this instance and
is in-line with the example here https://github.com/rack/rack/blob/master/example/protectedlobster.rb

index.html

@@ -47,7 +47,7 @@ <h1>Rack: a Ruby Webserver Interface</h1>
                     ['200', {'Content-Type' =&gt; 'text/html'}, ['A barebones rack app.']]
                 end
 
-                Rack::Handler::WEBrick.run app
+                Rack::Server.start :app => app, :Port => 9000
             </pre>
             <p>
                 Or, you can use the <span class="code">rackup</span> command line