6prof: more accurate usage message.
6prof: more accurate usage message.

R=r, r2
CC=golang-dev
http://codereview.appspot.com/2323042

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go