Fix lesson 2 readme (reactjs#175)

Author: fvgs

lessons/02-rendering-a-route/README.md

@@ -10,7 +10,7 @@ That's not going to display anything until we configure a route.
 
 Open up `index.js` and
 
-1. import `Router` and `Route`
+1. import `Router`, `Route`, and `hashHistory`
 2. render a `Router` instead of `App`
 
 ```js