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>

Documentation/core-api/dma-api-howto.rst

@@ -8,7 +8,7 @@ Dynamic DMA mapping Guide
 
 This is a guide to device driver writers on how to use the DMA API
 with example pseudo-code.  For a concise description of the API, see
-DMA-API.txt.
+Documentation/core-api/dma-api.rst.
 
 CPU and DMA addresses
 =====================