fix bugs in Singleton implementation

Author: XinliWang

code/Coding-Interviews/src/com/todorex/offer2/Singleton3.java

@@ -32,7 +32,7 @@ public static Singleton3 getSingleton() {
         if (singleton == null) {
             synchronized (Singleton3.class) {
                 if (singleton == null) {
-                    return new Singleton3();
+                    singleton = new Singleton3();
                 }
             }
         }

notes/实现Singleton模式.md

@@ -159,7 +159,7 @@ public class Singleton3 {
         if (singleton == null) {
             synchronized (Singleton3.class) {
                 if (singleton == null) {
-                    return new Singleton3();
+                    singleton = new Singleton3();
                 }
             }
         }