Fix broken links in docs

src/handling.rs

@@ -103,7 +103,7 @@ where
     /// Any requests passed to the handler will be directly handed down to the handler,
     /// with the request as the first argument.  The response provided will be sent to the
     /// requester.  If the handler panics or returns an [`Err`], this will be logged, and
-    /// the requester will be sent a [`SERVER_ERROR`](Response::server_error()).
+    /// the requester will be sent a [`TEMPORARY FAILURE`](Response::temporary_failure()).
     fn from(handler: H) -> Self {
         Self::FnHandler(
             Box::new(move|req| Box::pin((handler)(req)) as HandlerResponse)