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: 04bf7ac646ab ("dm: add documentation for dm-vdo target")
Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Mikulas Patocka <[e-mail]>

Documentation/admin-guide/device-mapper/vdo.rst

···
 .. SPDX-License-Identifier: GPL-2.0-only
 
+======
 dm-vdo
 ======