docs/kokr: Link memory-barriers.txt to rst

This commit links Korean translation of 'memory-barriers.txt' in the
translations index rst file as the original version is linked in
'Documentation/staging/index.rst'.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Link: https://lore.kernel.org/r/20210117100931.9347-4-sj38.park@gmail.com


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