Update GAME_BALANCE.txt

As part of the critter-consolidation, updating per #6346.
sinkHOLE

This makes sinkholes leave holes (as in, fall down a level) after you
trigger them.  Still doesn't do anything if critters hit 'em, but it
makes a bit more sense than leaving a pit in the square you escaped to
and the sinkhole remaining unsunk.
Merge pull request #6347 from KA101/sinkhole-fix

Sinkhole fix
Merge pull request #6332 from KA101/Re-Balance

Update GAME_BALANCE.txt