Makes manually assigned id example simpler to understand.

The old version was still valid but a little confusing.

Original pull request #3865
Signed-off-by: w3n9 <w3n9@foxmail.com>

Commit message changed by Jens Schauder

src/main/antora/modules/ROOT/pages/jpa/entity-persistence.adoc

@@ -40,7 +40,7 @@ public abstract class AbstractEntity<ID> implements Persistable<ID> {
     return isNew; <2>
   }
 
-  @PrePersist <3>
+  @PostPersist <3>
   @PostLoad
   void markNotNew() {
     this.isNew = false;