rate: change doc for ReserveN to reflect its signature

Change-Id: I97299d5a90b4273f42cb992bcf05b3c4786a2ac7
Reviewed-on: https://go-review.googlesource.com/32239
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: songgao

rate/rate.go

@@ -199,9 +199,10 @@ func (lim *Limiter) Reserve() *Reservation {
 // The Limiter takes this Reservation into account when allowing future events.
 // ReserveN returns false if n exceeds the Limiter's burst size.
 // Usage example:
-//   r, ok := lim.ReserveN(time.Now(), 1)
-//   if !ok {
+//   r := lim.ReserveN(time.Now(), 1)
+//   if !r.OK() {
 //     // Not allowed to act! Did you remember to set lim.burst to be > 0 ?
+//     return
 //   }
 //   time.Sleep(r.Delay())
 //   Act()