🐛 Use 'Spell' instead of 'RSpell' for clearing used staff charges

Readied spell can be swapped to something else in between the cast and the hit frame, resulting in incorrect removal.

This is how currently belt and inventory scrolls are checked so we are now using a consistent strategy.

Source/inv.cpp

@@ -1960,7 +1960,7 @@ void UseStaffCharge(Player &player)
 {
 	auto &staff = player.InvBody[INVLOC_HAND_LEFT];
 
-	if (!CanUseStaff(staff, player._pRSpell))
+	if (!CanUseStaff(staff, player._pSpell))
 		return;
 
 	staff._iCharges--;