minor #9091 Update built_in_web_server.rst (MacDada, javiereguiluz)

This PR was merged into the 4.0 branch.

Discussion
----------

Update built_in_web_server.rst

Kernel cannot extend Kernel ;)

Commits
-------

eefbe7f Removed some contents unnecessary when using Flex
004526b Update built_in_web_server.rst

Author: javiereguiluz

setup/built_in_web_server.rst

@@ -22,34 +22,12 @@ and enable the server bundle.
 Installing the Web Server Bundle
 --------------------------------
 
-First, execute this command:
+Move into your project directory and run this command:
 
 .. code-block:: terminal
 
     $ cd your-project/
-    $ composer require --dev symfony/web-server-bundle
-
-Then, enable the bundle in the kernel of the application::
-
-    // src/Kernel.php
-    class Kernel extends Kernel
-    {
-        public function registerBundles()
-        {
-            $bundles = array(
-                // ...
-            );
-
-            if ('dev' === $this->getEnvironment()) {
-                // ...
-                $bundles[] = new Symfony\Bundle\WebServerBundle\WebServerBundle();
-            }
-
-            // ...
-        }
-
-        // ...
-    }
+    $ composer require server --dev
 
 Starting the Web Server
 -----------------------