pgsql: Fix error message typo.

Fix error message typo.

4e5920e6de8 added an ereport call with the primary error message
starting with upper case, which is prohibited by our error message
style guide. This commit fixes it.

Author: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Tatsuo Ishii <ishii@postgresql.org>
Discussion: /messages/by-id/8BACA715-B9B6-479D-9153-C05F05482664@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ab3023ad1e685987ee1b5e7f28e0635296464cf5

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