sort: fix typo, was a mixup between identifiers 'unsorted' and 'data'

Change-Id: If9ad8ae663f007efe43cc35631713565fa754e93
Reviewed-on: https://go-review.googlesource.com/93237
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/sort/sort_test.go

@@ -194,7 +194,7 @@ func BenchmarkSortString1K_Slice(b *testing.B) {
 func BenchmarkStableString1K(b *testing.B) {
 	b.StopTimer()
 	unsorted := make([]string, 1<<10)
-	for i := 0; i < len(data); i++ {
+	for i := range unsorted {
 		unsorted[i] = strconv.Itoa(i ^ 0x2cc)
 	}
 	data := make([]string, len(unsorted))