move the SO question URL

singleton/src/main/java/com/iluwatar/App.java

@@ -5,6 +5,8 @@ package com.iluwatar;
  * Singleton pattern ensures that the class (IvoryTower) can have only one
  * existing instance and provides global access to that instance.
  *
+ * http://stackoverflow.com/questions/70689/what-is-an-efficient-way-to-implement-a-singleton-pattern-in-java
+ *
  */
 public class App {
 

singleton/src/main/java/com/iluwatar/EnumIvoryTower.java

@@ -3,7 +3,6 @@ package com.iluwatar;
 /**
  *
  * Enum Singleton class.
- * http://stackoverflow.com/questions/70689/what-is-an-efficient-way-to-implement-a-singleton-pattern-in-java
  *
  */
 public enum EnumIvoryTower {