Fix typo and outdated information in README.barrier
README.barrier didn't seem to get the memo when atomics were added. Fix
that.
Author: Tatsuo Ishii, David Rowley
Discussion: https://postgr.es/m/
20210516.211133.
2159010194908437625.t-ishii%40sraoss.co.jp
Backpatch-through: 9.6, oldest supported release