Fix doc (fixup #24031)

Author: Manishearth

src/doc/trpl/concurrency.md

@@ -245,10 +245,9 @@ We now call `clone()` on our `Arc`, which increases the internal count. This
 handle is then moved into the new thread. Let's examine the body of the
 thread more closely:
 
-```
+```rust
 # use std::sync::{Arc, Mutex};
 # use std::thread;
-#
 # fn main() {
 #     let data = Arc::new(Mutex::new(vec![1u32, 2, 3]));
 #     for i in 0..2 {
@@ -258,7 +257,6 @@ thread::spawn(move || {
     data[i] += 1;
 });
 #     }
-#
 #     thread::sleep_ms(50);
 # }
 ```