PreStart to Protected (#6889)

Co-authored-by: Hamza Amjad <hamza.amjad@revolvinggames@gmail.com>

docs/articles/actors/receive-actor-api.md

@@ -155,7 +155,7 @@ This strategy is typically declared inside the actor in order to have access to
 The remaining visible methods are user-overridable life-cycle hooks which are described in the following:
 
 ```csharp
-public override void PreStart()
+protected override void PreStart()
 {
 }