Fix the Heroku Button URL (#192)

The previous URL didn't specify an explicit template parameter, which means
Dashboard has to rely upon the referrer, which is no longer passed on, meaning
the Button had stopped working.

Author: edmorley

README.md

@@ -14,7 +14,7 @@ This application supports the [Getting Started with Python on Heroku](https://de
 
 Alternatively, you can deploy it using this Heroku Button:
 
-[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy)
+[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/heroku/python-getting-started)
 
 For more information about using Python on Heroku, see these Dev Center articles: