docs: dma: update a reference to a moved document

Documentation/DMA-API.txt has moved to Documentation/core-api/dma-api.rst

Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Message-ID: <20231101070201.4066998-1-lizhijian@fujitsu.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>