go/build: fix doc typo
go/build: fix doc typo

go/build section "Build Constraints", first paragraph said:
  "... they must be appear near the top of the file ..."
fixed to:
  "... they must appear near the top of the file ..."

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6329060

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go