Fix command that sets APP_URL in Heroku.md

[johnberberich]

The heroku apps:info --shell command outputs the app's URL in an attribute named web-url, not web_url (with a hyphen, not an underscore).

[johnberberich]

I should also note that the original command set Heroku's APP_URL environment variable to an empty string.

Having an invalid APP_URL results in the following error when running rake lazy_fetch:

Running rake lazy_fetch on heroku-app-1234... up, run.2751
** Building /js/application.js...
** Building /css/application.css...
rake aborted!
Connection refused - connect(2)
/app/Rakefile:27:in `block in <top (required)>'
Tasks: TOP => lazy_fetch
(See full trace by running task with --trace)

[swanson]

Thanks!

[johnberberich]

No problem! I've been sitting on an empty Stringer feed for over a week now, and this morning I figured I'd dig into it to correct the problem. Luckily it was just configuration on my end. Anyway, thank you for creating and maintaining the project!