doc: update Effective Go for template API change.
doc: update Effective Go for template API change.

The original change 3378041 was submitted in December 2010:
template: change the signature of formatters for future development.

R=adg
CC=golang-dev
http://codereview.appspot.com/3862041

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