refactor(02_Dependent_Type_Theory.org): emphasize the difference between declaring and defining
refactor(02_Dependent_Type_Theory.org): emphasize the difference between declaring and defining

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