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 <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/driver-api/dma-buf.rst

···
    :internal:
 
 Indefinite DMA Fences
-~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~
 
 At various times &dma_fence with an indefinite time until dma_fence_wait()
 finishes have been proposed. Examples include: