docs: nvdimm: use ReST notation for subsection

docs: nvdimm: use ReST notation for subsection

The ACPI Device Specific Methods (_DSM) paragraph is intended to be a
subsection of the Submit Checklist Addendum section. Dan Williams however
used Markdown notation for this subsection, which does not parse as
intended in a ReST documentation.

Change the markup to ReST notation, as described in the Specific
guidelines for the kernel documentation section in
Documentation/doc-guide/sphinx.rst.

Fixes: 47843401e3a0 ("libnvdimm, MAINTAINERS: Maintainer Entry Profile")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20200118153620.8276-1-lukas.bulwahn@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>