Add heading for exercise 3

Exercises/exercise-4/Solution.hs

@@ -54,6 +54,9 @@ length' ls = aux
             x : xs -> aux $ map (\y -> (+1) x ) xs
 -- length' und aux sind nicht endrekursiv
 
+--------------------
+-- Exercise 3
+--------------------
 -- sieve :: ( a -> a -> Bool ) -> [ a ] -> [ a ]
 -- sieve pred xs = case xs of
 --     [] -> []