doc/gopher: fix typo in README
doc/gopher: fix typo in README

LGTM=josharian
R=golang-codereviews, josharian
CC=golang-codereviews
https://codereview.appspot.com/80220043

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