dma-buf: Add "dma-buf" to title of documentation

To make it easier to find the dma-buf documentation when looking through
tables-of-contents etc., put the name "dma-buf" in the title.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20230129115917.1229529-1-j.neuschaefer@gmx.net


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