math/rand: update comment to avoid use of ^ for exponentiation

Fixes #35920

Change-Id: I1a4d26c5f7f3fbd4de13fc337de482667d83c47f
Reviewed-on: https://go-review.googlesource.com/c/go/+/209758
Reviewed-by: Alberto Donizetti <[email protected]>

Author: bradfitz

src/math/rand/rand.go

@@ -298,7 +298,7 @@ var _ *rngSource = globalRand.src.(*lockedSource).src
 // Seed uses the provided seed value to initialize the default Source to a
 // deterministic state. If Seed is not called, the generator behaves as
 // if seeded by Seed(1). Seed values that have the same remainder when
-// divided by 2^31-1 generate the same pseudo-random sequence.
+// divided by 2³¹-1 generate the same pseudo-random sequence.
 // Seed, unlike the Rand.Seed method, is safe for concurrent use.
 func Seed(seed int64) { globalRand.Seed(seed) }