[slides][properties] add link to exercise in slides

slides/properties.md

@@ -496,3 +496,8 @@ class Assembler {
    }
 };
 ```
+
+# Exercise
+## Exercise
+
+* Go to [Properties exercise](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-course/-/tree/master/exercises/exercise-properties)