docs: dma-buf: fix some warnings

Fix those warnings:

	Documentation/driver-api/dma-buf.rst:182: WARNING: Title underline too short.

	Indefinite DMA Fences
	~~~~~~~~~~~~~~~~~~~~

	Documentation/driver-api/dma-buf.rst:88: WARNING: Unknown target name: "fence poll support".

The first one is due to a shorter markup. The second one is
because the chapter name was wrong.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/b2bc0bc88eb913635cfece13cc9f6eff7668d333.1599660067.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/driver-api/dma-buf.rst

@@ -85,7 +85,7 @@ consider though:
 - Memory mapping the contents of the DMA buffer is also supported. See the
   discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.
 
-- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for
+- The DMA buffer FD is also pollable, see `Implicit Fence Poll Support`_ below for
   details.
 
 Basic Operation and Device DMA Access