Use twig instead of templating engine to render about page

This restores compatibility with Contao 4.8 without changing default
configuration of the symfony framework bundle.

src/Bundle/Resources/config/services.yml

@@ -136,5 +136,5 @@ services:
   Netzmacht\Contao\Leaflet\Backend\Action\AboutAction:
     public: true
     arguments:
-      - '@templating'
+      - '@twig'
       - '%kernel.project_dir%'