pgsql: Fix grammar in error message

Fix grammar in error message

While at it, make ellipses formatting consistent when describing SQL statements.

Ekaterina Kiryanova and Alexander Lakhin

Reviewed by myself and Álvaro Herrera
Discussion: https://www.postgresql.org/message-id/eed5cec0-a542-53da-6a5e-7789c6ed9817%40postgrespro.ru
Backpatch only the grammar fix to v15

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7beda87b6a4e5eae66f06df313581f7f53cab813

Modified Files
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src/backend/commands/subscriptioncmds.c | 4 ++--
src/backend/postmaster/bgworker.c       | 2 +-
src/backend/rewrite/rewriteHandler.c    | 2 +-
src/test/regress/expected/with.out      | 4 ++--
src/test/regress/sql/with.sql           | 2 +-
5 files changed, 7 insertions(+), 7 deletions(-)