The plan9.bell-labs.com site has fallen into disrepair.
We'll instead use the site maintained by contributor David du Colombier.

Fixes #14233

Change-Id: I0c702e5d3b091cccd42b288ea32f34d507a4733d
Reviewed-on: https://go-review.googlesource.com/19240Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>