book: linkify the rustup.rs URL
book: linkify the rustup.rs URL

BUG=None
TEST=mdbook build # test that rustup ref becomes a clickable link

Change-Id: Ieea42a363130ca42cea6db9133e2a8de94a1d275
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3691970
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm