runtime: fix comment

Fixes #14259

Change-Id: I23fedec0eb85ae28e56bc24539bc864674856130
Reviewed-on: https://go-review.googlesource.com/19318Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/time.go

@@ -16,7 +16,7 @@ type timer struct {
 	i int // heap index
 
 	// Timer wakes up at when, and then at when+period, ... (period > 0 only)
-	// each time calling f(now, arg) in the timer goroutine, so f must be
+	// each time calling f(arg, now) in the timer goroutine, so f must be
 	// a well-behaved function and not block.
 	when   int64
 	period int64