testing: update package docs to use ResetTimer() instead of StopTimer/StartTimer()

R=golang-dev, rsc, r
CC=golang-dev
https://golang.org/cl/6591051

src/pkg/testing/testing.go

@@ -27,15 +27,14 @@
 //
 // The benchmark package will vary b.N until the benchmark function lasts
 // long enough to be timed reliably.  The output
-//     testing.BenchmarkHello    10000000    282 ns/op
+//     BenchmarkHello    10000000    282 ns/op
 // means that the loop ran 10000000 times at a speed of 282 ns per loop.
 //
 // If a benchmark needs some expensive setup before running, the timer
-// may be stopped:
+// may be reset:
 //     func BenchmarkBigLen(b *testing.B) {
-//         b.StopTimer()
 //         big := NewBig()
-//         b.StartTimer()
+//         b.ResetTimer()
 //         for i := 0; i < b.N; i++ {
 //             big.Len()
 //         }