runtime: typo in comment about timer type's fields

[ggaaooppeenngg]

https://github.com/golang/go/blob/master/src/runtime/time.go#L19
In this line should
// each time calling f(now, arg) in the timer goroutine, so f must be
be replace with // each time calling f(arg, now) in the timer goroutine, so f must be, it seems seq is the timestamp of the timer, and it is passed as now in f.

[gopherbot]

CL https://golang.org/cl/19318 mentions this issue.