Doc correction

The doc misleadingly says that the third value of the task-status-array contains the "# of tiles remaining", but according to [this line](https://github.com/GeoWebCache/geowebcache/blob/6399412/geowebcache/core/src/main/java/org/geowebcache/seed/TileBreeder.java#L434) the "expected remaining time in seconds" is contained in that value.

Please review and feel free to merge.