Correct timer type in "Delay and Timeout using Monotonics"

The example in `lm3s6965/examples/async-timeout.rs` uses `Mono` as the monotonic timer type, so it's confusing that the second example switches to using `Systick`.

Author: thejpster

book/en/src/by-example/delay.md

@@ -21,7 +21,7 @@ A _software_ task can `await` the delay to expire:
 #[task]
 async fn foo(_cx: foo::Context) {
     ...
-    Systick::delay(100.millis()).await;
+    Mono::delay(100.millis()).await;
     ...
 }