math/rand/v2: use a doc link for crypto/rand

math/rand/v2: use a doc link for crypto/rand

It's easier to go look at its documentation when there's a link.

Change-Id: Iad6c1aa1a3f4b9127dc526b4db473239329780d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/563255
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>