Add Unraid borgmatic installation link to docs.

docs/how-to/set-up-backups.md

@@ -96,7 +96,7 @@ installing borgmatic:
  * [macOS (via MacPorts)](https://ports.macports.org/port/borgmatic/)
  * [NixOS](https://search.nixos.org/packages?show=borgmatic&sort=relevance&type=packages&query=borgmatic)
  * [Ansible role](https://github.com/borgbase/ansible-role-borgbackup)
- * [virtualenv](https://virtualenv.pypa.io/en/stable/)
+ * [Unraid](https://unraid.net/community/apps?q=borgmatic#r)
 
 
 ## Hosting providers