docs: DMA-API-HOWTO.txt: fix an unmarked code block

When building with Sphinx, it would produce this warning:

    docs/Documentation/DMA-API-HOWTO.rst:222: WARNING: Definition list ends without a blank line; unexpected unindent.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/DMA-API-HOWTO.txt

@@ -212,7 +212,7 @@ The standard 64-bit addressing device would do something like this::
 
 If the device only supports 32-bit addressing for descriptors in the
 coherent allocations, but supports full 64-bits for streaming mappings
-it would look like this:
+it would look like this::
 
 	if (dma_set_mask(dev, DMA_BIT_MASK(64))) {
 		dev_warn(dev, "mydev: No suitable DMA available\n");