fix solution of [Medium] groupingOfNumbers problem (#21)

* fixed solution of [Medium] groupingOfNumbers problem

* Update MediumNumbersProblemSolution.java

Author: The-Wardaddy

src/test/java/com/github/streams/practice/b_medium/numbers/MediumNumbersProblemSolution.java

@@ -91,6 +91,6 @@ public static double meanOfNumbers(List<Integer> input) {
   }
 
   public static Map<Integer, List<Integer>> groupingOfNumbers(List<Integer> randomIntegers) {
-    return randomIntegers.stream().collect(Collectors.groupingBy(e -> e / 10));
+    return randomIntegers.stream().collect(Collectors.groupingBy(e -> (e / 10) * 10));
   }
 }