sync: fix typo in benchmark name

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/118860043

src/pkg/sync/pool_test.go

@@ -136,7 +136,7 @@ func BenchmarkPool(b *testing.B) {
 	})
 }
 
-func BenchmarkPoolOverlflow(b *testing.B) {
+func BenchmarkPoolOverflow(b *testing.B) {
 	var p Pool
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {