dma-buf: Fix kerneldoc formatting

I wanted to look up something and noticed the hyperlink doesn't work.
While fixing that also noticed a trivial kerneldoc comment typo in the
same section, fix that too.

Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201204200242.2671481-1-daniel.vetter@ffwll.ch