Inline variables printed with println! and friends (#315)

The course follows the style of inlining variable names where possible
in `println!` statements.

src/concurrency/channels/bounded.md

@@ -21,7 +21,7 @@ fn main() {
     thread::sleep(Duration::from_millis(100));
 
     for msg in rx.iter() {
-        println!("Main: got {}", msg);
+        println!("Main: got {msg}");
     }
 }
 ```