fix:修改一些错字。

source/_posts/quick-start-akka.md

@@ -47,7 +47,7 @@ public interface Command {}
 
 public class Request<T> implements Command {
     private T payload;
-    private ActorOf<Command> sender;
+    private ActorRef<Command> sender;
 
     // getter和setter先省略了
 }