Update dependency_injection.rst

Add missing use of `Symfony\Component\HttpFoundation\RequestStack`

Author: flip111

create_framework/dependency_injection.rst

@@ -10,9 +10,10 @@ to it::
     namespace Simplex;
 
     use Symfony\Component\EventDispatcher\EventDispatcher;
-    use Symfony\Component\Routing;
     use Symfony\Component\HttpFoundation;
+    use Symfony\Component\HttpFoundation\RequestStack;
     use Symfony\Component\HttpKernel;
+    use Symfony\Component\Routing;
 
     class Framework extends HttpKernel\HttpKernel
     {