Update README.md fluidsystem

exercises/exercise-fluidsystem/README.md

@@ -53,7 +53,7 @@ The `TwoP` _TypeTag_ can be found in the `2p/model.hh` header:
 #include <dumux/porousmediumflow/2p/model.hh>
 ```
 
-while the `BoxModel` _TypeTag_ can be found in the `box/properties.hh` header:
+while the `BoxModel` _TypeTag_ can be found in the `discretization/box.hh` header:
 
 ```c++
 // The box discretization