Update README.md

README.md

@@ -19,8 +19,8 @@ concurrency.
 - [x] systematic exception propagation from `fork`ed tasks
 - [x] unbounded concurrency with the `future` primitive
 - [x] asynchronous waiting on `Future` and `Promise` objects
+- [x] mutexes (with asynchronous blocking)
 - [ ] channels (single producer, single consumer)
-- [ ] mutexes (with asynchronous blocking)
 - [ ] asynchronous I/O
 - [ ] task cancellation