docs/maintenance-upgrading: indent "either" commands

improves readability

docs/maintenance-upgrading-services.md

@@ -8,8 +8,8 @@ To upgrade services:
 
 - update your playbook directory and all upstream Ansible roles (defined in the `requirements.yml` file) using:
 
-- either: `just update`
-- or: a combination of `git pull` and `just role` (or `make roles`)
+  - either: `just update`
+  - or: a combination of `git pull` and `just role` (or `make roles`)
 
 - take a look at [the changelog](../CHANGELOG.md) to see if there have been any backward-incompatible changes that you need to take care of