Update server-rendering.md

Author: ryanflorence

docs/guides/server-rendering.md

@@ -21,7 +21,7 @@ var routes = (
 app.use(function (req, res) {
   // pass in `req.path` and the router will immediately match
   Router.run(routes, req.path, function(Handler) {
-    var content = <Handler/>;
+    var content = React.renderToString(<Handler/>);
     res.render('main', {content: content});
   });
 });