Fix small problems in osm boundaries files

[antoine-de]

fix 2 small format errors in the file:

• add missing "relation" tag for buenos aires
• overrides shouldn't be nested in admin_level, but should be at the top level

[albarrentine]

Thanks! I still need to investigate the builds - they've been spotty since moving the releases to Github, though only on the CI, doesn't seem to be an issue when building locally. It might have something to do with using the Github API unauthenticated to get the file timestamps. In general it might be better to keep track of the model versions using files in the repo instead.