docs: Fix type in actor children example (#3274)

site/docs/02-fundamentals/03-actors.mdx

@@ -82,7 +82,7 @@ const parent = new ex.Actor({...});
 
 const child = new ex.Actor({...});
 
-actor.addChild(child);
+parent.addChild(child);
 ```
 
 1. Paper doll assemblies - Moving a parent body actor and having child actor arms, legs, head, etc. pieces move.