ipv4: fix typo
ipv4: fix typo

Change-Id: Iaec7505bea3ea29cd58147b6057f03a4c7f0861c
Reviewed-on: https://go-review.googlesource.com/29694
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/net