update hello_world tutorial example to 0.2.1 API

book/src/getting_started/hello_world.md

@@ -7,7 +7,7 @@ copy and paste the following code:
 ```rust
 extern crate amethyst;
 
-use amethyst::{Application, Duration, State, Trans};
+use amethyst::engine::{Application, Duration, State, Trans};
 
 struct HelloWorld;