pgsql: Avoid splitting errmsg string to span multiple lines

Avoid splitting errmsg string to span multiple lines

The error message being fixed was added in 6185c9737c.

While at it, add an "a" to the sentence.

Reported-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20240322.095149.895185546948714852.horikyota.ntt%40gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/085e759e9da77b7f5e859d23f72653f36277a053

Modified Files
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src/backend/parser/parse_expr.c                  | 4 +---
src/test/regress/expected/sqljson_queryfuncs.out | 4 ++--
2 files changed, 3 insertions(+), 5 deletions(-)