doc/codewalk: update urlpoll to use time.Duration.
doc/codewalk: update urlpoll to use time.Duration.

R=adg
CC=golang-dev
http://codereview.appspot.com/5545061

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang