Adds minor code formatting changes

exercises/series/src/example/java/Series.java

     private List<Integer> digits;
 
     Series(String string) {
-        this.digits =
-                Arrays.stream(string.split(("")))
+        this.digits = Arrays.stream(string.split(""))
                         .map(Integer::parseInt)
                         .collect(Collectors.toList());
         this.digitsSize = this.digits.size();