Update doc example of actix middleware

Hi,

since v2 this middleware documentation example won't compile anymore due to lack of Req type declaration: https://actix.rs/docs/middleware/

I've been fiddling around with it to make my middleware compatible, but couldn't figure this one out:
``` rust
impl Transform for MyMiddleware
where
S: Service, Error = Error>,
S::Future: 'static,
B: 'static,
{
```
the compiler complains about
`S: Service, Error = Error>,`
because

```
wrong number of type arguments: expected 1, found 0
expected 1 type argument
```