docs(hertz): fix shell output in example code

[gaspardruan]

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docs: Documentation only changes

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Here's the actual log output.

func (engine *Engine) Shutdown(ctx context.Context) (err error) {
// ...
	opt := engine.GetOptions()
	hlog.SystemLogger().Infof("Begin graceful shutdown, wait at most %s ...", opt.ExitWaitTimeout)
// ...
}

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