Update HTTP server example for reactphp/http v1.6.0

Author: clue

theme/homepage.html.twig

@@ -42,11 +42,7 @@
             {% markdown %}
 ```php
 $http = new React\Http\HttpServer(function (Psr\Http\Message\ServerRequestInterface $request) {
-    return new React\Http\Message\Response(
-        200,
-        array(
-            'Content-Type' => 'text/plain'
-        ),
+    return React\Http\Message\Response::plaintext(
         "Hello World!\n"
     );
 });