os: Fix comment in generated signal_unix.go file.
os: Fix comment in generated signal_unix.go file.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5013042
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go