Fix example of 'Hello World' app

Added missing parameters into the callback of $app->get('/', ...

README.md

@@ -74,7 +74,7 @@ $app = AppFactory::create();
 $app->addErrorMiddleware(true, true, true);
 
 // Add routes
-$app->get('/', function () {
+$app->get('/', function (Request $request, Response $response) {
     $response->getBody()->write('<a href="/hello/world">Try /hello/world</a>');
     return $response;
 });