updated code to follow brief's format better and tested save and open functionality

src/main/java/cvbuilder/model/UserGroup.java

@@ -5,7 +5,7 @@ import java.util.ArrayList;
 public class UserGroup {
 
     private static UserGroup instance;
-    private ArrayList<User> users;
+    private final ArrayList<User> users;
 
     private UserGroup() {
         users = new ArrayList<>();