Auto merge of rust-lang#30221 - thyrgle:concurrency_doc, r=alexcrichton

The example code in the Channels subsection of the rust book give warnings about

    unused result which must be used, #[warn(unused_must_use)] on by default

Added a small pattern match to resolve those warnings.

Author: bors

src/doc/book/concurrency.md

@@ -295,12 +295,12 @@ fn main() {
             let mut data = data.lock().unwrap();
             *data += 1;
 
-            tx.send(());
+            tx.send(()).unwrap();
         });
     }
 
     for _ in 0..10 {
-        rx.recv();
+        rx.recv().unwrap();
     }
 }
 ```
@@ -324,7 +324,7 @@ fn main() {
         thread::spawn(move || {
             let answer = i * i;
 
-            tx.send(answer);
+            tx.send(answer).unwrap();
         });
     }