docs/memory-barriers.txt: Fix references for DMA*.txt files

Commit 985098a0 ("docs: fix references for DMA*.txt files") missed
fixing memory-barriers.txt file.  This commit applies the change to the
file.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Link: https://lore.kernel.org/r/20200829082607.3146-2-sj38.park@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>