fix cursed spellbook reference in iron's apothic invaders datapack mod

[Linguardium]

fixes:

[06:45:44] [main/ERROR] [placebo/]: Failed parsing gear_sets file apotheosis:cultist.
[06:45:44] [main/ERROR] [placebo/]: Underlying Exception: 
io.netty.handler.codec.CodecException: Codec failure for type gear_sets, message: Failed to read non-optional item id irons_spellbooks:cursed_doll_spellbook

irons_spellbooks:cursed_doll_spellbook should be irons_spellbooks:cursed_doll_spell_book