reverse-string: add argument to initial solution (#1070)

* changed initial reverse string stub to include argument

* change reverse string initial solution argument to match ref solution

exercises/reverse-string/src/main/java/ReverseString.java

 class ReverseString {
-  
-    String reverse(String stringToReverse) {
+
+    String reverse(String inputString) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }