Minor fix to mutex example

Presumably `N` was supposed to be used in both places.

src/libstd/sync/mutex.rs

@@ -61,7 +61,7 @@ use sys_common::poison::{self, TryLockError, TryLockResult, LockResult};
 /// let data = Arc::new(Mutex::new(0));
 ///
 /// let (tx, rx) = channel();
-/// for _ in 0..10 {
+/// for _ in 0..N {
 ///     let (data, tx) = (data.clone(), tx.clone());
 ///     thread::spawn(move || {
 ///         // The shared state can only be accessed once the lock is held.