fix(03_Propositions_and_Proofs): typo
fix(03_Propositions_and_Proofs): typo

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