fix(13): change finset.induction to Lean text
fix(13): change finset.induction to Lean text

Closes #186

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