kokr/memory-barriers.txt: Fix obsolete link to atomic_ops.txt

Obsolete links to atomic_ops.txt exist in ko_KR/memory-barriers.txt
though the file has moved to core-api/atomic_ops.rst.  This commit fixes
the obsolete links.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>