flag: failf documentation nit
flag: failf documentation nit

"standard output" should have been "standard error".  Sorry about that..

R=adg
CC=golang-dev
http://codereview.appspot.com/4240088

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

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