The provided instructions for how to add RPM package repo fail on Fedora 43:
```
Unknown argument "--add-repo" for command "config-manager". Add "--help" for more information about the arguments.
```
The [docs](https://forgejo.org/docs/latest/user/packages/rpm/#configuring-the-package-registry-using-dnf5) contain a different command, which does work.
This changes instructions to match docs.
Co-authored-by: sarge <ephemeralsarge@example.com>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11478
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: sarge <sarge@noreply.codeberg.org>
Co-committed-by: sarge <sarge@noreply.codeberg.org>
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