icmp: fix a typo
icmp: fix a typo

Change-Id: Icd29cd883dab0c916e2285d6053074e97771b840
Reviewed-on: https://go-review.googlesource.com/21833
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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