Correctly shutdown wrapped Executor in PausableChannelEventExecutor.shutdown()

Motivation:

PausableChannelEventExecutor().shutdown() used to call unwrap().terminationFuture() instead of unwrap.shutdown().
This error was reported by @xfrag in a comment to a commit message [1]. Tyvm.

Modifications:

PausableChannelEventExecutor.shutdown() now correctly invokes unwrap().shutdown() instead of unwrap().terminationFuture().

Result:

Correct code.

[1] 220660e351 (commitcomment-8489643)

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

@@ -200,7 +200,7 @@ abstract class PausableChannelEventExecutor implements PausableEventExecutor, Ch
     @Override
     @Deprecated
     public void shutdown() {
-        unwrap().terminationFuture();
+        unwrap().shutdown();
     }
 
     @Override