dm-vdo: Promote dm-vdo title to title heading

dm-vdo docs currently has no explicit title heading but instead there
are multiple section headings as top-level heading. As such, these
sections are rendered as titles and inflates number of entries in the
toctree index.

Promote the first section heading ("dm-vdo") to title heading.

Fixes: 04bf7ac6 ("dm: add documentation for dm-vdo target")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>