Documentation: fix dma-buf.rst underline length warning

Documentation: fix dma-buf.rst underline length warning

/home/rdunlap/lnx/lnx-59-rc2/Documentation/driver-api/dma-buf.rst:182: WARNING: Title underline too short.
Indefinite DMA Fences
~~~~~~~~~~~~~~~~~~~~

Fixes: 72b6ede73623 ("dma-buf.rst: Document why indefinite fences are a bad idea")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1b22d4c3-4ea5-c633-9e35-71ce65d8dbcc@infradead.org