time: incorrect comment/documentation in time.Location

Hi,

In:
https://github.com/golang/go/blob/master/src/time/zoneinfo.go#L24

```
// if cacheStart <= t <= cacheEnd,
// lookup can return cacheZone.
```

This should be:

```
// if cacheStart <= t < cacheEnd,
```

For actual code:

https://github.com/golang/go/blob/master/src/time/zoneinfo.go#L120
