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>
diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
index 1a721d0..db48c6fd 100644
--- a/Documentation/DMA-API-HOWTO.txt
+++ b/Documentation/DMA-API-HOWTO.txt
@@ -147,7 +147,7 @@
 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