Fix typo in MessageToMessageDecoder api docs. (netty#8638)

Motivation:

We had some typo (most likely caused by copy-and-paste) in the api docs which should be fixed.

Modifications:

Replace encoder by decoder word.

Result:

Correct apidocs.

codec/src/main/java/io/netty/handler/codec/MessageToMessageDecoder.java

@@ -107,7 +107,7 @@ public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception
 
     /**
      * Decode from one message to an other. This method will be called for each written message that can be handled
-     * by this encoder.
+     * by this decoder.
      *
      * @param ctx           the {@link ChannelHandlerContext} which this {@link MessageToMessageDecoder} belongs to
      * @param msg           the message to decode to an other one