crypto/internal/rand: fix typo to cryptotest.SetGlobalRandom

Change-Id: Id24147c24c9e511b7463dd82ace9edbbcecc2977
Reviewed-on: https://go-review.googlesource.com/c/go/+/779882
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>