x/net/ipv4: update package example
x/net/ipv4: update package example

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/178800043

cesanta/goxnet,w4ngyi/net,subuk/net,cmars/tools,yangxianjun/net,DramaFever/golang.org-x-net,mckinney-subgraph/net,muzining/net,zabawaba99/net,DramaFever/golang.org-x-net,cmars/tools,zabawaba99/net,chenzlabs/net,kiip/net,w4ngyi/net,veliamsli/net,golang/net,dafyddcrosby/net,gomini/net,chenzlabs/net,foreversmart/net,subuk/net,raboof/net,cmars/tools,gomini/net,yangxianjun/net,raboof/net,flowlo/go-net,flowlo/go-net,albertjin/golang-net,albertjin/golang-net,veliamsli/net,dafyddcrosby/net,muzining/net,foreversmart/net,cesanta/goxnet,cmars/tools,mckinney-subgraph/net,kiip/net