Give Mummies a +2 Spellcasting aptitude

To partially imitate Ogres' old aptitude pattern of high spellcasting but low
spell schools, which was a fairly interesting feature lost by their rework -
Mummies also have a similar thing already with their +0 Fighting and -2 weapon
aptitudes.

crawl-ref/source/aptitudes.h

@@ -304,7 +304,7 @@ static const species_skill_aptitude species_skill_aptitudes[] =
     APT(SP_MUMMY,           SK_TRAPS,         UNUSABLE_SKILL),
 #endif
     APT(SP_MUMMY,           SK_UNARMED_COMBAT, -2),
-    APT(SP_MUMMY,           SK_SPELLCASTING,   -1),
+    APT(SP_MUMMY,           SK_SPELLCASTING,    2),
     APT(SP_MUMMY,           SK_CONJURATIONS,   -2),
     APT(SP_MUMMY,           SK_HEXES,          -1),
     APT(SP_MUMMY,           SK_CHARMS,         -2),