GH-213 - Specify proper transactional propagation type in docs.

The default propagation type of @Transactional is Propagation.REQUIRED. That, however would reuse the already running committed transaction which is not the correct behavior.

Related ticket: #80.

src/docs/asciidoc/40-events.adoc

@@ -87,7 +87,7 @@ To run a transactional event listener in a transaction itself, it would need to
 class InventoryManagement {
 
   @Async
-  @Transactional
+  @Transactional(propagation = Propagation.REQUIRES_NEW)
   @TransactionalEventListener
   void on(OrderCompleted event) { /* … */ }
 }