apt repositories: fix typo for getting the default unknown text

apt repositories: fix typo for getting the default unknown text

Could lead to a type error with classifyOrigin when there is a
repository that doesn't have an InRelease file and cannot be detected
as Debian/Proxmox origin from its URL. For me, it triggered with the
element.io repository after changing to bookworm (which currently
doesn't exist yet) and running apt update.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
(cherry picked from commit 78be60a079695b0306b6530eb0f757fe4125e472)
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>