etl: remove public access modifier from etl

exercises/etl/.meta/src/reference/java/Etl.java

 import java.util.List;
 import java.util.Map;
 
-public class Etl {
-    public Map<String, Integer> transform(Map<Integer, List<String>> old) {
+class Etl {
+    Map<String, Integer> transform(Map<Integer, List<String>> old) {
         final Map<String, Integer> result = new HashMap<String, Integer>();
         for (Map.Entry<Integer, List<String>> e : old.entrySet()) {
             for (String s : e.getValue()) {

exercises/etl/src/main/java/Etl.java

 import java.util.List;
 import java.util.Map;
 
-public class Etl {
-    public Map<String, Integer> transform(Map<Integer, List<String>> old) {
+class Etl {
+    Map<String, Integer> transform(Map<Integer, List<String>> old) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
 }