fmt: remove an obsolete reference to os.ErrorString in a comment
fmt: remove an obsolete reference to os.ErrorString in a comment

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5229057
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go