fix(chapter5): fix typo
fix(chapter5): fix typo

cjmazey/fork_leanprover_tutorial,Hodapp87/cs6460_project,cjmazey/fork_leanprover_tutorial,Hodapp87/cs6460_project,fpvandoorn/tutorial,soonhokong/lean-tutorial,soonhokong/lean-tutorial,fpvandoorn/tutorial,leanprover/tutorial,avigad/tutorial,avigad/tutorial,soonhokong/lean-tutorial,leanprover/tutorial,leanprover/tutorial,cjmazey/fork_leanprover_tutorial,Hodapp87/cs6460_project,fpvandoorn/tutorial,avigad/tutorial