runtime/extern.go: clear a small error in a comment.
runtime/extern.go: clear a small error in a comment.

R=iant, rsc
CC=golang-dev
http://codereview.appspot.com/1741058

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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