Replace data.clone() by Arc::clone(&data) in mutex doc.

Arc::clone(&from) is considered as more idiomatic because it conveys more explicitly the meaning of the code.

src/libstd/sync/mutex.rs

@@ -69,7 +69,7 @@
 ///
 /// let (tx, rx) = channel();
 /// for _ in 0..N {
-///     let (data, tx) = (data.clone(), tx.clone());
+///     let (data, tx) = (Arc::clone(&data), tx.clone());
 ///     thread::spawn(move || {
 ///         // The shared state can only be accessed once the lock is held.
 ///         // Our non-atomic increment is safe because we're the only thread