beer-song: update reference solution to package-private

exercises/beer-song/.meta/src/reference/java/BeerSong.java

-public class BeerSong {
-    public String verse(int number) {
+class BeerSong {
+    String verse(int number) {
         switch (number) {
             case 0:
                 return "No more bottles of beer on the wall, no more bottles of beer.\nGo to the store and buy some more, 99 bottles of beer on the wall.\n\n";
         }
     }
 
-    public String sing(int start, int stop) {
+    String sing(int start, int stop) {
         StringBuilder songOutput = new StringBuilder();
 
         for (int i=start; i>=stop; i--) {
         return songOutput.toString();
     }
 
-    public String singSong() {
+    String singSong() {
         return sing(99,0);
     }
 }