Added missing Dotenv use

Author: JohnstonCode

migration.rst

@@ -239,6 +239,7 @@ could look something like this::
     use App\Kernel;
     use App\LegacyBridge;
     use Symfony\Component\Debug\Debug;
+    use Symfony\Component\Dotenv\Dotenv;
     use Symfony\Component\HttpFoundation\Request;
 
     require dirname(__DIR__).'/vendor/autoload.php';