Update tiler.rb - Fix zoom level directories names

Autozoom directory names would always start at 20 and count down.
So at `--autozoom 4` the sub-directories would be `20`, `19`, `18`, `17`.

With this change autozoom directories start at autozoom-1 and count down.
So at `--autozoom 4` the sub-directories will *now* be `3`, `2`, `1`, `0`.