fix actix-service doc error
@@ -52,7 +52,7 @@ pub use self::transform::{apply, Transform};
/// type Request = u8;
/// type Response = u64;
/// type Error = MyError;
-/// type Future = Pin<Box<Future<Output=Result<Self::Response, Self::Error>>>;
+/// type Future = Pin<Box<Future<Output=Result<Self::Response, Self::Error>>>>;
///
/// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> { ... }
///
