Simplify async channel example

This removes a nested import (which I find hard to read) and removes
an unnecessary type annotation.

src/concurrency/async-control-flow/channels.rs

@@ -1,7 +1,7 @@
-use tokio::sync::mpsc::{self, Receiver};
+use tokio::sync::mpsc;
 
-async fn ping_handler(mut input: Receiver<()>) {
-    let mut count: usize = 0;
+async fn ping_handler(mut input: mpsc::Receiver<()>) {
+    let mut count = 0;
 
     while let Some(_) = input.recv().await {
         count += 1;