incorrect path for gentoo prefix

I use gentoo prefix:

```dosini
[DEFAULT]
main-repo = gentoo

[gentoo]
location = /var/db/repos/gentoo
sync-uri = tar+https://github.com/gentoo-mirror/gentoo/archive/stable.tar.gz

[binpkgs]
location = /var/cache/binpkgs
repo-type = binpkg-v1
```

So the repos.conf of pkgcore is incorrect.