In the tutorial, Michael says:
Unfortunately, the resulting page is an error; as of Rails 4.0, for technical reasons the default Rails page doesn’t work on Heroku. The good news is that the error will go away (in the context of the full sample application) when we add a root route in Section 5.3.2. - 1.4.3 Heroku deployment, step two
You might haven't created the top page yet in section 2.3.5. If it's your case, setting proper route to existing page would do.
Also you can refer to Heroku dev center :
Rails 4 no longer has a static index page in production. When you’re using a new app, there will not be a root page in production, so we need to create one. We will first create a controller called welcome for our home page to live: - Getting Started with Rails 4.x on Heroku