Cannot use when fork name differs from upstream repo name

**Describe the bug**

When your github fork has a different name than upstream repo, you cannot use decapcms.

**To Reproduce**

I'm using github backend with open authoring in https://github.com/OCA/docs

This repo was before named https://github.com/moduon/oca-docs while it was a prototype. Some users already tested the prototype with Decap CMS and [they had forks](https://github.com/OCA/docs/forks) named `oca-docs` because of that:

Now, when these users try to [use decap-cms in the new site](https://oca-docs.netlify.app/admin/), they cannot because their fork isn't named `docs` but `oca-docs`.

**Expected behavior**

They can use open authoring

**Screenshots**

Do login. The, click on fork:

Then, the error:

**Applicable Versions:**

- Decap CMS version: 3.0.0
- Git provider: GitHub
- OS: Ubuntu
- Browser version: Chrome.

**CMS configuration**

https://github.com/OCA/docs/blob/b79cd4097f188f19a4182bf6ec203e4c6d104eef/static/admin/config.yml

**Additional context**