doc: fix warning treated as an error

Typical error:

```
Warning, treated as error:
/home/jenkins-build/build/workspace/ceph-ansible-docs-pull-requests/docs/source/day-2/upgrade.rst:2:Title underline too short.
```
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

docs/source/day-2/upgrade.rst

 Upgrading the ceph cluster
--------------------
+--------------------------
 
 ceph-ansible provides a playbook in ``infrastructure-playbooks`` for upgrading a Ceph cluster: ``rolling_update.yml``.