modify link on "Deploy to Heroku" button according to the repository's name change

Clicking the "Deploy to Heroku" button on the README,
I found an error by Heroku:

> There was a problem with the app.json file found at "https://github.com/mietek/gitit":
No app.json located in the repo URL provided. Are you sure the URL is correct and that an app.json file exists in the project root directory?