put slides in order as taught at the course

slides/index.md

@@ -26,8 +26,8 @@ subtitle: Overview
 
 * [Property System](./properties.html)
 * [Constitutive models (material)](./materialsystem.html)
-* [Implementing a new model in DuMu^x^](./model.html)
 * [New Dune module](./dunemodule.html)
+* [Implementing a new model in DuMu^x^](./model.html)
 
 ##