html/template: fix doc typo
html/template: fix doc typo

Fixes issue 4950.

R=golang-dev, peter
CC=golang-dev
https://codereview.appspot.com/7415046

tv42/old-go,apparentlymart/golang,gnanderson/go,tv42/old-go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang