Update broken "channels" link (#2146)

Stumbled upon another broken link. Courtesy fix 🫡

Author: snazzybytes

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

@@ -39,7 +39,7 @@ async fn main() {
 - Change the channel size to `3` and see how it affects the execution.
 
 - Overall, the interface is similar to the `sync` channels as seen in the
-  [morning class](concurrency/channels.md).
+  [morning class](../channels.md).
 
 - Try removing the `std::mem::drop` call. What happens? Why?