Documentation: DMA-API: fix two typos

This patch fixes two typos, a missing "e" and dma-api/driver_filter was
incorrectly typed dma-api/driver-filter.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
[jc: fixed obvious language typos on the way in]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>