Update PersistAsync to match docs. (#5736)

This is supposed to be the example for persistAsync, but it doesn't get used at all.

https://getakka.net/articles/persistence/event-sourcing.html#relaxed-local-consistency-requirements-and-high-throughput-use-cases

I changed the code to match what is here:

https://doc.akka.io/docs/akka/current/persistence.html#relaxed-local-consistency-requirements-and-high-throughput-use-cases

src/core/Akka.Docs.Tests/Persistence/PersistentActor/PersistAsync.cs

@@ -27,9 +27,8 @@ protected override void OnCommand(object message)
                 if (message is string c)
                 {
                     Sender.Tell(c);
-                    Persist($"evt-{c}-1", e => Sender.Tell(e));
-                    Persist($"evt-{c}-2", e => Sender.Tell(e));
-                    DeferAsync($"evt-{c}-3", e => Sender.Tell(e));
+                    PersistAsync($"evt-{c}-1", e => Sender.Tell(e));
+                    PersistAsync($"evt-{c}-2", e => Sender.Tell(e));
                 }
             }
         }