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 <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: mvdan

src/math/rand/v2/rand.go

@@ -14,7 +14,7 @@
 //
 // This package's outputs might be easily predictable regardless of how it's
 // seeded. For random numbers suitable for security-sensitive work, see the
-// crypto/rand package.
+// [crypto/rand] package.
 package rand
 
 import (