doc: mention go/printer instead of container/vector in effective go
doc: mention go/printer instead of container/vector in effective go

Fixes issue 1876.

R=golang-dev, bradfitz, gri, r
CC=golang-dev
http://codereview.appspot.com/4538093

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