fix: tutorials refers to non-existent heroku app

[dopry]

Describe the bug
The tutorial talks about a heroku app at the very beginning. This heroku app doesn't seem to exist and doesn't seem to be used later in the tutorial. We need to review the tutorial for accuracy and remove all references to the heroku app.

To Reproduce
read https://django-oauth-toolkit.readthedocs.io/en/latest/tutorial/tutorial_01.html#start-your-app

Expected behavior
The tutorial is accurate

Version
2.3.0