fix(13): wrap zero_add with namespace playground
fix(13): wrap zero_add with namespace playground

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