Add missing unwrap()s

`tx.send()` returns a `Result` which must be used otherwise the compiler produces warnings.

content/tokio/tutorial/channels.md

@@ -175,11 +175,13 @@ async fn main() {
     let tx2 = tx.clone();
 
     tokio::spawn(async move {
-        tx.send("sending from first handle").await;
+        // send() returns a `Result` which cannot be ignored.
+        tx.send("sending from first handle").await.unwrap();
     });
 
     tokio::spawn(async move {
-        tx2.send("sending from second handle").await;
+        // send() returns a `Result` which cannot be ignored.
+        tx2.send("sending from second handle").await.unwrap();
     });
 
     while let Some(message) = rx.recv().await {