twelve-days: remove public's from starter implementation

exercises/twelve-days/src/main/java/TwelveDays.java

-public class TwelveDays {
-    public String verse(int verseNumber) {
+class TwelveDays {
+    String verse(int verseNumber) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
 
-    public String verses(int startVerse, int endVerse) {
+    String verses(int startVerse, int endVerse) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
 }