Update ChannelOutboundBuffer.java (#9863)

Motivation:

fix comment of ChannelOutboundBuffer.CHANNEL_OUTBOUND_BUFFER_ENTRY_OVERHEAD

Modifications:

6 (NOT 8) reference fields is right

Result:
Correct comment

transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java

@@ -54,7 +54,7 @@ import static java.util.Objects.requireNonNull;
 public final class ChannelOutboundBuffer {
     // Assuming a 64-bit JVM:
     //  - 16 bytes object header
-    //  - 8 reference fields
+    //  - 6 reference fields
     //  - 2 long fields
     //  - 2 int fields
     //  - 1 boolean field