      doc: fix typo in Concurrency Patterns article
      doc: fix typo in Concurrency Patterns article

Fixes issue 3178.

R=golang-dev, iant
CC=golang-dev
http://codereview.appspot.com/6506064
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go