Clarify IndexLink and punctuation fix (reactjs#198)

* Clarify that we want to use IndexLink instead of NavLink
* Apostrophe was missing in `let's`

Author: Sudheer Satyanarayana

lessons/09-index-links/README.md

@@ -23,7 +23,7 @@ index route is rendered.
 
 ## IndexLink
 
-First lets use the `IndexLink`
+First let's use the `IndexLink` instead of `NavLink`
 
 ```js
 // App.js