Fix placement of subterranean part of the HWS

The elevator (in this case in the Hazardous Waste Sarcophagus) always moves exactly 2 z-levels.

But the subterranean part of the HWS was only one z-level down, the next z-level (where the elevator would lead to) was still solid rock and therefor the elevator refuses to go there.
This restores the old placement where the lower part is 2 z-levels down. Only works on new overmaps.

Fixes http://smf.cataclysmdda.com/index.php?topic=8316.0