template: refer HTML users to html/template
template: refer HTML users to html/template

Fixes issue 2963.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5650051

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