Replace 'mutex' with 'lock' in RwLock documentation

When copying the documentation for `clear_poison` from Mutex, not every occurence of 'mutex' was replaced with 'lock'.

library/std/src/sync/rwlock.rs

@@ -380,7 +380,7 @@ impl<T: ?Sized> RwLock<T> {
     ///
     /// If the lock is poisoned, it will remain poisoned until this function is called. This allows
     /// recovering from a poisoned state and marking that it has recovered. For example, if the
-    /// value is overwritten by a known-good value, then the mutex can be marked as un-poisoned. Or
+    /// value is overwritten by a known-good value, then the lock can be marked as un-poisoned. Or
     /// possibly, the value could be inspected to determine if it is in a consistent state, and if
     /// so the poison is removed.
     ///
@@ -397,7 +397,7 @@ impl<T: ?Sized> RwLock<T> {
     ///
     /// let _ = thread::spawn(move || {
     ///     let _lock = c_lock.write().unwrap();
-    ///     panic!(); // the mutex gets poisoned
+    ///     panic!(); // the lock gets poisoned
     /// }).join();
     ///
     /// assert_eq!(lock.is_poisoned(), true);