Documentation: DMA API: fix a typo in a function name

Correct the typo, the wrongly typed function does not exist.

Fixes: 6c9c6d63 ("dma-debug: New interfaces to debug dma mapping errors")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>