ipv4: update example for the use of dual stack listener
ipv4: update example for the use of dual stack listener

Fixes golang/go#13612.

Change-Id: Ie224c1018fb4ceb013476f586000801ce447956e
Reviewed-on: https://go-review.googlesource.com/17798
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cesanta/goxnet,cmars/tools,DramaFever/golang.org-x-net,albertjin/golang-net,DramaFever/golang.org-x-net,albertjin/golang-net,cesanta/goxnet,cmars/tools,cmars/tools,golang/net,cmars/tools