fix(08_Building_Theories_and_Proofs): adjust example to reflect recent changes in Lean
fix(08_Building_Theories_and_Proofs): adjust example to reflect recent changes in Lean

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