adds missing re-adding of origin to repo cleaning (#874)

The missing bit was [identified in the Codeberg e. V. member forum](https://forum.codeberg.org/d/265-problems-regarding-the-instructions-in-reducing-the-size-of-a-repository), this just adds the remote that gets removed back in.

Reviewed-on: https://codeberg.org/Codeberg/Documentation/pulls/874
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

content/git/reduce-size.md

@@ -129,6 +129,7 @@ to undo in case anything goes wrong.
 
    ```shell
    git config unset remote.origin.mirror
+   git remote add origin git@codeberg.org:your_user_name/your_repo.git
    git push origin --force 'refs/heads/*'
    git push origin --force 'refs/tags/*'
    git push origin --force 'refs/replace/*'