docs: DMA-API-HOWTO: add a missing "="

The === line is shorter than the section title.

As this will generate a warning with rst build, fix it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DMA-API-HOWTO.txt

@@ -147,7 +147,7 @@ networking subsystems make sure that the buffers they use are valid
 for you to DMA from/to.
 
 DMA addressing capabilities
-==========================
+===========================
 
 By default, the kernel assumes that your device can address 32-bits of DMA
 addressing.  For a 64-bit capable device, this needs to be increased, and for