Fix borg repository URL format

Reference: https://borgbackup.readthedocs.io/en/stable/usage/general.html#repository-urls

Otherwise, we'd get:

> /etc/borgmatic.d/config.yaml: Remote repository paths without ssh:// syntax are deprecated. Interpreting "user@hostname:matrix" as "ssh://user@hostname/./matrix"

docs/configuring-playbook-backup-borg.md

@@ -40,7 +40,7 @@ Minimal working configuration (`inventory/host_vars/matrix.DOMAIN/vars.yml`) to
 ```yaml
 matrix_backup_borg_enabled: true
 matrix_backup_borg_location_repositories:
- - USER@HOST:REPO
+ - ssh://USER@HOST/./REPO
 matrix_backup_borg_storage_encryption_passphrase: "PASSPHRASE"
 matrix_backup_borg_ssh_key_private: |
   -----BEGIN OPENSSH PRIVATE KEY-----