Fix docs, response instead of req in ExampleRedditAPI.md

Author: Evgeny Samsonov

docs/advanced/ExampleRedditAPI.md

@@ -65,7 +65,7 @@ function fetchPosts(subreddit) {
   return dispatch => {
     dispatch(requestPosts(subreddit))
     return fetch(`http://www.reddit.com/r/${subreddit}.json`)
-      .then(req => req.json())
+      .then(response => response.json())
       .then(json => dispatch(receivePosts(subreddit, json)))
   }
 }