Fix typo (#92)

src/async/README.md

@@ -1,6 +1,6 @@
 # Working with `async`
 
-So far we’ve only been working with synchronous users interfaces: You provide some input,
+So far we’ve only been working with synchronous user interfaces: You provide some input,
 the app immediately processes it and updates the interface. This is great, but is a tiny
 subset of what web applications do. In particular, most web apps have to deal with some kind of asynchronous data loading, usually loading something from an API.