docs: DMA-API-HOWTO.txt: fix an unmarked code block

When building with Sphinx, it would produce this warning:

    docs/Documentation/DMA-API-HOWTO.rst:222: WARNING: Definition list ends without a blank line; unexpected unindent.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
1 file changed