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] https://github.com/netty/netty/commit/220660e351b2a22112b19c4af45e403eab1f73ab#commitcomment-8489643