dmaengine: doc: Properly indent metadata title

The newly added metadata documentation title was not properly indented
resulting in doc build break:

  Sphinx parallel build error:
  docutils.utils.SystemMessage: /linux/Documentation/driver-api/dmaengine/client.rst:155: (SEVERE/4) Unexpected section title.

  Optional: per descriptor metadata
  ---------------------------------

Fix this by doing the right indent

Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Fixes: 7d083ae9 ("dmaengine: doc: Add sections for per descriptor metadata support")
Link: https://lore.kernel.org/r/20200128125032.1650816-1-vkoul@kernel.org


Signed-off-by: Vinod Koul <vkoul@kernel.org>