doc: fix typo in Effective Go
doc: fix typo in Effective Go

Fixes issue 2120.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/4810061

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang