fix typos in various places

src/libstd/sync/mod.rs

@@ -97,7 +97,7 @@
 //! - A **multiprocessor** system executing multiple hardware threads
 //!   at the same time: In multi-threaded scenarios, you can use two
 //!   kinds of primitives to deal with synchronization:
-//!   - [memory fences] to ensure memory accesses are made visibile to
+//!   - [memory fences] to ensure memory accesses are made visible to
 //!   other CPUs in the right order.
 //!   - [atomic operations] to ensure simultaneous access to the same
 //!   memory location doesn't lead to undefined behavior.

src/libstd/sync/once.rs

@@ -290,8 +290,8 @@ impl Once {
     }
 
     /// Returns true if some `call_once` call has completed
-    /// successfuly. Specifically, `is_completed` will return false in
-    /// the following situtations:
+    /// successfully. Specifically, `is_completed` will return false in
+    /// the following situations:
     ///   * `call_once` was not called at all,
     ///   * `call_once` was called, but has not yet completed,
     ///   * the `Once` instance is poisoned