Docs: Correct flags for Docker Compose

[JoshuaVandaele]

Closes #6411

Changes proposed in this pull request:

• Updated documentation

How to test the feature manually:

Follow the docker compose instructions.

Pull request checklist:

• [x] clear commit messages
• [x] code manually tested
• [ ] unit tests written (optional if too hard)
• [x] documentation updated

[Alkarex]

Thanks for the PR, but see explanation https://github.com/FreshRSS/FreshRSS/issues/6411#issuecomment-2088941040