Seriously reduces pebbles' base range.

A 9mm round has base range of 14, so these having a range of 30 was
rather ridiculous.
Fixes https://github.com/CleverRaven/Cataclysm-DDA/issues/8601.
Merge pull request #8604 from Rivet-the-Zombie/pebble-range-fix

Seriously reduces pebbles' base range.