doc: fix padding on h1 tags
doc: fix padding on h1 tags

See /doc/codewalk/sharemem/ for an example of how it's broken.

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/4742044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go