pgsql: Fix typo and outdated information in README.barrier

Fix typo and outdated information in README.barrier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

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/010429eb1f77a6221f6c89b21b6b0c7820446201

Modified Files
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src/backend/storage/lmgr/README.barrier | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)