[MINOR] a minor fix for the comments of a method in RPC Dispatcher

## What changes were proposed in this pull request?

a minor fix for the comments of a method in RPC Dispatcher

## How was this patch tested?

existing unit tests

Author: CodingCat <zhunansjtu@gmail.com>

Closes #11738 from CodingCat/minor_rpc.

core/src/main/scala/org/apache/spark/rpc/netty/Dispatcher.scala

@@ -136,7 +136,7 @@ private[netty] class Dispatcher(nettyEnv: NettyRpcEnv) extends Logging {
    * Posts a message to a specific endpoint.
    *
    * @param endpointName name of the endpoint.
-   * @param createMessageFn function to create the message.
+   * @param message the message to post
    * @param callbackIfStopped callback function if the endpoint is stopped.
    */
   private def postMessage(