Update lib/tileup/tiler.rb to fix autozoom directory names.

Autozoom directory names start from 20 and count down.
So at `--autozoom 4` the directory names would be `20`, `19`, `18`, `17`.

Now, autozoom directory names will start at autozoom - 1.
So at `--autozoom 4` the directory names will be `3`, `2`, `1`, `0`.