fix spelling mistake

Author: Jon Mountjoy

README.md

@@ -39,7 +39,7 @@ If you want to work on this application we’d love your pull requests and ticke
 Deploy to Heroku
 ----------------
 
-Click the buttom below to set up this sample app on Heroku:
+Click the button below to set up this sample app on Heroku:
 
 [![Deploy](https://www.herokucdn.com/deploy/button.png)](https://heroku.com/deploy)