dma-api-howto: typo fix

Stumbled upon a typo while reading the doc, here's a fix.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/af1505348a67981f63ccff4e3c3d45b686cda43f.1680864874.git.mst@redhat.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>