math/rand: fix comment for Shuffle

Shuffle panics if n < 0, not n <= 0. The comment for the (*Rand).Shuffle
function is already accurate.

Change-Id: I073049310bca9632e50e9ca3ff79eec402122793
Reviewed-on: https://go-review.googlesource.com/63750Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/rand/rand.go

@@ -346,7 +346,7 @@ func Float32() float32 { return globalRand.Float32() }
 func Perm(n int) []int { return globalRand.Perm(n) }
 
 // Shuffle pseudo-randomizes the order of elements using the default Source.
-// n is the number of elements. Shuffle panics if n <= 0.
+// n is the number of elements. Shuffle panics if n < 0.
 // swap swaps the elements with indexes i and j.
 func Shuffle(n int, swap func(i, j int)) { globalRand.Shuffle(n, swap) }