doc: update signal.Notify example to use buffered channel
doc: update signal.Notify example to use buffered channel

This if follow up of CL 274332.

Updates #9399.

Change-Id: Ic6dd534dc18227a799cbb9577979f2285596b825
Reviewed-on: https://go-review.googlesource.com/c/go/+/274393
Trust: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
Run-TryBot: Cuong Manh Le <54b4457f1dff6823379654eb5867596497ba782f@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go