go/doc: use correct escaper for URL

go/doc: use correct escaper for URL

Despite the name, URL escaping is for a small subpiece of the URL only.
This particular URL is being emitted in an <a href=""> tag and in that
context it should be HTML escaped, not URL escaped.

In addition to fixing a bug, this change cuts a dependency chain
from go/doc to net, clearing the way for use of cgo
(which imports go/doc) during the compilation of package net.

R=gri
CC=golang-dev
https://golang.org/cl/1960050