doc: fix an article
doc: fix an article

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/91770050

wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old