math/bits: update reference to debruijn paper

math/bits: update reference to debruijn paper

The old link no longer works.

Fixes #70684

Change-Id: I8711ef7d5721bf20ef83f5192dd0d1f73dda6ce1
Reviewed-on: https://go-review.googlesource.com/c/go/+/633775
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>