Fix Responder documentation: remove an outdated reference to 'Future'.

core/lib/src/response/responder.rs

@@ -22,8 +22,7 @@ use crate::request::Request;
 ///
 /// # Return Value
 ///
-/// A `Responder` returns a `Future` whose output type is an `Ok(Response)` or
-/// an `Err(Status)`:
+/// A `Responder` returns an `Ok(Response)` or an `Err(Status)`:
 ///
 ///   * An `Ok` variant means that the `Responder` was successful in generating
 ///     a `Response`. The `Response` will be written out to the client.