fix: broken readme demo link

[josephschito]

Description

Current demo link points to a down Heroku app, updated with what I found here.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Manual review steps

1. Check if the new link is ok.

[codeclimate]

Code Climate has analyzed commit 969193c and detected 0 issues on this pull request.

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[Paul-Bob]

Thank you @josephschito !