Fix unterminated single quote (#1565)

README.md

@@ -65,7 +65,7 @@ $ rails generate administrate:install
 Restart your server, and visit http://localhost:3000/admin
 to see your new dashboard in action.
 
-For more detailed instructions or to make it work with Rails API-only applications, please go through the ['Getting Started` guide](https://administrate-prototype.herokuapp.com/getting_started).
+For more detailed instructions or to make it work with Rails API-only applications, please go through the ['Getting Started' guide](https://administrate-prototype.herokuapp.com/getting_started).
 
 If your apps uses Sprockets 4, you'll need to add Administrate's assets to your `manifest.js` file. To do this, add these two lines to the file: