Make Holder class final.

Author: phipag

powertools-idempotency/powertools-idempotency-core/src/main/java/software/amazon/lambda/powertools/idempotency/Idempotency.java

@@ -143,7 +143,7 @@ private void setPersistenceStore(BasePersistenceStore persistenceStore) {
         this.persistenceStore = persistenceStore;
     }
 
-    private static class Holder {
+    private static final class Holder {
         private static final Idempotency instance = new Idempotency();
     }