gitlab_runner: Fix incorrect description in libvirt-executor-update-base-image.timer

Fixes: 7ea1eb29 ("gitlab_runner: Refactor libvirt-executor")

roles/gitlab_runner/files/libvirt-executor-update-base-image.timer

 [Unit]
-Description=Run libvirt-executor-vm-template.service daily
+Description=Run libvirt-executor-update-base-image.service daily
 
 [Timer]
 OnCalendar=daily