math/rand, math/rand/v2: rename receiver variables

According to the https://go.dev/wiki/CodeReviewComments#receiver-names

Change-Id: Ib8bc57cf6a680e5c75d7346b74e77847945f6939
Reviewed-on: https://go-review.googlesource.com/c/go/+/568635


Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>