doc: fix warning treated as an error
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 <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible