fix(09_Type_Classes): clarify "arbitrary" example
fix(09_Type_Classes): clarify "arbitrary" example

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