fix(03): "import classical" ==> "open classical"
fix(03): "import classical" ==> "open classical"

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