math/bits: fix example for OnesCount64

Erroneously called OnesCount instead of OnesCount64

Change-Id: Ie877e43f213253e45d31f64931c4a15915849586
Reviewed-on: https://go-review.googlesource.com/53410
Reviewed-by: Chris Broadfoot <[email protected]>

Author: campoy

src/math/bits/example_test.go

@@ -71,7 +71,7 @@ func ExampleOnesCount32() {
 
 func ExampleOnesCount64() {
 	fmt.Printf("%b\n", 14)
-	fmt.Println(bits.OnesCount(14))
+	fmt.Println(bits.OnesCount64(14))
 	// Output:
 	// 1110
 	// 3