godoc: enable visibility of element linked from URL hash

Expands the example when visiting a URL such as
http://golang.org/pkg/net/http/#example_Hijacker

Fixes issue 5212.

R=golang-dev, bradfitz, adg
CC=golang-dev
https://codereview.appspot.com/8378043

Committer: Andrew Gerrand <adg@golang.org>