rate: make function comment match function name

Change-Id: I603ca855eac736e90d2ba2bb7ce61981a4a91918
Reviewed-on: https://go-review.googlesource.com/c/time/+/638776
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: cuishuang

rate/rate_test.go

@@ -192,7 +192,7 @@ func (tt *testTime) advance(dur time.Duration) {
 	tt.advanceUnlocked(dur)
 }
 
-// advanceUnlock advances the fake time, assuming it is already locked.
+// advanceUnlocked advances the fake time, assuming it is already locked.
 func (tt *testTime) advanceUnlocked(dur time.Duration) {
 	tt.cur = tt.cur.Add(dur)
 	i := 0