pgsql: Fix error message wording

Fix error message wording

The originals are ambiguous and a bit out of style.

Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/202412141243.efesjyyvzxsz@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0e5b14410e2bb54d191bbb7bb21a674ccabe768e

Modified Files
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src/backend/parser/parse_expr.c                      | 5 +++--
src/interfaces/ecpg/test/expected/sql-sqljson.stderr | 6 +++---
src/test/regress/expected/sqljson.out                | 2 +-
3 files changed, 7 insertions(+), 6 deletions(-)