fix(13): improve wording
fix(13): improve wording

And fix typo "a new elements"
leanprover/tutorial,Hodapp87/cs6460_project,fpvandoorn/tutorial,fpvandoorn/tutorial,Hodapp87/cs6460_project,leanprover/tutorial,soonhokong/lean-tutorial,Hodapp87/cs6460_project,soonhokong/lean-tutorial,fpvandoorn/tutorial,avigad/tutorial,avigad/tutorial,avigad/tutorial,soonhokong/lean-tutorial,leanprover/tutorial