Middleware example should be async

[rougeth]

There are two examples of middlewares at the documentation, a function and a class middlewares. The function middleware should be async and the class middleware doesn't actually work.

[jessicapaz]

Hey @rougeth, can I take this one?

[rougeth]

@jessicapaz sure! Thanks.

[mvellasco]

@jessicapaz Are you still working on this issue? If you're not please let know.