ipv4: fix a typo
ipv4: fix a typo

Change-Id: I1e4133d1988cb745d465b478bcdf01594304e48a
Reviewed-on: https://go-review.googlesource.com/21834
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

albertjin/golang-net,golang/net,albertjin/golang-net