doc: increase h3 size

This makes h3 and h4 more distinct, and h2 and h3 the same size.
The h2 and h3 styles may be distinguished h2's background styles.

This means that almost all text on the site is either 16px, 20px,
or 24px. (with a smattering of 14px)

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5754095

doc/style.css

@@ -71,7 +71,7 @@ h2 {
 	padding: 2px 5px;
 }
 h3 {
-	font-size: 18px;
+	font-size: 20px;
 }
 h3,
 h4 {