AtomicInteger to replace synchronized (#543)

With AtomicInteger, synchronized is no longer needed.

docs/book/Appendix-Low-Level-Concurrency.md

@@ -1053,7 +1053,7 @@ public class
 AtomicSerialNumbers extends SerialNumbers {
   private AtomicInteger serialNumber =
     new AtomicInteger();
-  public synchronized int nextSerialNumber() {
+  public int nextSerialNumber() {
     return serialNumber.getAndIncrement();
   }
   public static void main(String[] args) {