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
https://golang.org/cl/4742044

doc/all.css

@@ -135,7 +135,6 @@ h1#title {
   clear: both;
 }
 #content h1 {
-  margin-bottom: -0em;
   padding: 0;
 }
 #content h2 {