fix(04): typos
fix(04): typos

I believe the parentheses are necessary here, otherwise the exercise is
impossible.  (It amounts to showing r from just A -> r, knowing nothing
of A.)

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