Fixup code sample

This block is supposed to be a mirror of the block above it, but with
most lines commented out to focus on the inner function body. However,
a few lines were missing in the commented region.

Author: callahad

src/doc/trpl/concurrency.md

@@ -248,6 +248,7 @@ thread more closely:
 ```
 # 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 {
@@ -257,6 +258,8 @@ thread::spawn(move || {
     data[i] += 1;
 });
 #     }
+#
+#     thread::sleep_ms(50);
 # }
 ```