lib/godoc/example.html: htmlify the play code Fixes issue 4539.
lib/godoc/example.html: htmlify the play code
Fixes issue 4539.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/6942043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go