[model] Update code in doc

.patches/exercise-model/exercise-model.patch

 diff -ruN exercises/exercise-model/CMakeLists.txt exercises/solution/exercise-model/CMakeLists.txt
---- exercises/exercise-model/CMakeLists.txt	2024-07-17 11:27:28.798347462 +0200
-+++ exercises/solution/exercise-model/CMakeLists.txt	2024-07-17 11:27:28.802347549 +0200
+--- exercises/exercise-model/CMakeLists.txt	2024-07-17 14:21:07.400210015 +0200
++++ exercises/solution/exercise-model/CMakeLists.txt	2024-07-17 14:21:07.404210034 +0200
 @@ -3,5 +3,5 @@
  
  dune_symlink_to_source_files(FILES images params.input)
@@ -9,8 +9,8 @@ diff -ruN exercises/exercise-model/CMakeLists.txt exercises/solution/exercise-mo
 +dumux_add_test(NAME exercise_nonlineardiffusion_sol
                 SOURCES main.cc)
 diff -ruN exercises/exercise-model/main.cc exercises/solution/exercise-model/main.cc
---- exercises/exercise-model/main.cc	2024-07-17 11:27:28.878349199 +0200
-+++ exercises/solution/exercise-model/main.cc	2024-07-17 11:27:28.886349373 +0200
+--- exercises/exercise-model/main.cc	2024-07-17 14:21:07.400210015 +0200
++++ exercises/solution/exercise-model/main.cc	2024-07-17 14:21:07.404210034 +0200
 @@ -29,6 +29,8 @@
  #include <dumux/nonlinear/newtonsolver.hh>
  #include <dumux/assembly/fvassembler.hh>
@@ -148,7 +148,7 @@ diff -ruN exercises/exercise-model/main.cc exercises/solution/exercise-model/mai
  }// end main
 diff -ruN exercises/exercise-model/model.hh exercises/solution/exercise-model/model.hh
 --- exercises/exercise-model/model.hh	1970-01-01 01:00:00.000000000 +0100
-+++ exercises/solution/exercise-model/model.hh	2024-07-17 11:27:28.886349373 +0200
++++ exercises/solution/exercise-model/model.hh	2024-07-18 10:07:49.719993891 +0200
 @@ -0,0 +1,210 @@
 +// -*- mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 +// vi: set et ts=4 sw=4 sts=4:
@@ -361,7 +361,7 @@ diff -ruN exercises/exercise-model/model.hh exercises/solution/exercise-model/mo
 +
 +#endif
 diff -ruN exercises/exercise-model/README.md exercises/solution/exercise-model/README.md
---- exercises/exercise-model/README.md	2024-07-16 16:28:05.559708538 +0200
+--- exercises/exercise-model/README.md	2024-07-18 10:13:40.660815419 +0200
 +++ exercises/solution/exercise-model/README.md	1970-01-01 01:00:00.000000000 +0100
 @@ -1,195 +0,0 @@
 -# Exercise Model (DuMuX course)
@@ -416,11 +416,11 @@ diff -ruN exercises/exercise-model/README.md exercises/solution/exercise-model/R
 -and use this new type tag for specializing properties, i.e. within the properties namespace
 -
 -```c++
--namespace Dumux::Properties {
+-namespace Dumux::Properties::TTag {
 -
--// Defined properties, replace TTag::DiffusionModel by TTag::NonlinearDiffusionModel
+-// Defined properties, replace DiffusionModel by NonlinearDiffusionModel
 -
--} // end namespace Dumux::Properties
+-} // end namespace Dumux::Properties::TTag
 -```
 -
 -## Task 3: Set up the test case

exercises/exercise-model/README.md

@@ -50,11 +50,11 @@ Each model also needs to define a model type tag for setting model-specific prop
 and use this new type tag for specializing properties, i.e. within the properties namespace
 
 ```c++
-namespace Dumux::Properties {
+namespace Dumux::Properties::TTag {
 
-// Defined properties, replace TTag::DiffusionModel by TTag::NonlinearDiffusionModel
+// Defined properties, replace DiffusionModel by NonlinearDiffusionModel
 
-} // end namespace Dumux::Properties
+} // end namespace Dumux::Properties::TTag
 ```
 
 ## Task 3: Set up the test case