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: NMichael 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.orgSigned-off-by: NVinod Koul <vkoul@kernel.org>

Documentation/driver-api/dmaengine/client.rst

@@ -151,8 +151,8 @@ The details of these operations are:
      Note that callbacks will always be invoked from the DMA
      engines tasklet, never from interrupt context.
 
-  Optional: per descriptor metadata
-  ---------------------------------
+Optional: per descriptor metadata
+---------------------------------
   DMAengine provides two ways for metadata support.
 
   DESC_METADATA_CLIENT