Remove reference to MutexArc

Author: derekchiang

src/doc/tutorial.md

@@ -2155,7 +2155,7 @@ unless they contain references.
 
 * `Share` - Types that are *threadsafe*
 These are types that are safe to be used across several threads with access to
-a `&T` pointer. `MutexArc` is an example of a *sharable* type with internal mutable data.
+a `&T` pointer. `Mutex<T>` is an example of a *sharable* type with internal mutable data.
 
 * `'static` - Non-borrowed types.
 These are types that do not contain any data whose lifetime is bound to