Changed classes and methods to be package private

exercises/two-fer/.meta/src/reference/java/Twofer.java

-public class Twofer {
-    public String twofer(String name) {
+class Twofer {
+    String twofer(String name) {
         return "One for " + (name != null ? name : "you") + ", one for me.";
     }
-}
+}

exercises/two-fer/src/main/java/Twofer.java

-public class Twofer {
-    public String twofer(String name) {
+class Twofer {
+    String twofer(String name) {
         throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
     }
-}
+}