Update doc tests for Budget

tower/src/retry/budget.rs

@@ -41,9 +41,9 @@
 //! }
 //!
 //! impl<E> Policy<Req, Res, E> for RetryPolicy {
-//! type Future = future::Ready<Self>;
+//! type Future = future::Ready<()>;
 //!
-//! fn retry(&self, req: &mut Req, result: &mut Result<Res, E>) -> Option<Self::Future> {
+//! fn retry(&mut self, req: &mut Req, result: &mut Result<Res, E>) -> Option<Self::Future> {
 //! match result {
 //! Ok(_) => {
 //! // Treat all `Response`s as success,
@@ -60,12 +60,12 @@
 //! }
 //!
 //! // Try again!
-//! Some(future::ready(self.clone()))
+//! Some(future::ready(()))
 //! }
 //! }
 //! }
 //!
-//! fn clone_request(&self, req: &Req) -> Option<Req> {
+//! fn clone_request(&mut self, req: &Req) -> Option<Req> {
 //! Some(req.clone())
 //! }
 //! }