put exercises in order as taught at the course

exercises/README.md

@@ -44,16 +44,6 @@ This exercise guides you through the following tasks:
 
 In this exercise, you learn how to adjust the properties in order to use a customized local residual.
 
-
-### [:open_file_folder: Model](./exercise-model/README.md)
-
-This exercise covers the implementation of new models in DuMu<sup>x</sup>. You learn
-
-* how to design a nonlinear diffusion equation model from scratch (almost)
-* how to implement a local residual using a minimal model design
-* use DuMu<sup>x</sup> to denoise an image (!)
-
-
 ### [:open_file_folder: Fluid systems](./exercise-fluidsystem/README.md)
 
 This exercise covers the handling of phases and components in DuMu<sup>x</sup>. You learn
@@ -62,7 +52,6 @@ This exercise covers the handling of phases and components in DuMu<sup>x</sup>.
 * how to implement a new fluidsystem,
 * and how to change the wettability of the porous medium.
 
-
 ### [:open_file_folder: Dune module](./exercise-dunemodule/README.md)
 
 You learn how to
@@ -70,6 +59,14 @@ You learn how to
 * create a new dune module which depends on the dumux module,
 * create a new GitLab project.
 
+### [:open_file_folder: Model](./exercise-model/README.md)
+
+This exercise covers the implementation of new models in DuMu<sup>x</sup>. You learn
+
+* how to design a nonlinear diffusion equation model from scratch (almost)
+* how to implement a local residual using a minimal model design
+* use DuMu<sup>x</sup> to denoise an image (!)
+
 ### [:open_file_folder: Coupling free and porous medium flow](./exercise-coupling-ff-pm/README.md)
 
 This exercise is related to the [SFB1313 Project Area A](https://www.sfb1313.uni-stuttgart.de/research-areas/)