Reduce Searing Ray power scaling

Searing Ray is a fun and distinctive early-game spell. However, with
the current damage values, it remains surprisingly effective well
into Lair branches, reducing the need to branch out into new spells.

To increase variety, let's keep the min damage the same while reducing
the max damage about 25%, from 3d7 to 2d8. (Fewer dice should also
increase variance, which is nice.)

Author: PleasingFungus

crawl-ref/source/zap-data.h

@@ -1369,7 +1369,7 @@ _mon_hex_zap(ZAP_SLEEP, BEAM_SLEEP),
     ZAP_SEARING_RAY,
     "searing ray",
     50,
-    new dicedef_calculator<3, 3, 1, 12>,
+    new calcdice_calculator<2, 9, 1, 7>,
     new tohit_calculator<11, 1, 8>,
     nullptr,
     nullptr,