pgsql: Fix typos in nodeWindowAgg comments

Fix typos in nodeWindowAgg comments

One of them submitted by the author, with another one other
spotted during review so this fixes both.

Author: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAHewXN=eNx2oJ_hzxJrkSvy-1A5Qf45SM8pxERWXE+6RoZyFrw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3a872ddd64f908ac3dd65c544c2439ecbe737799

Modified Files
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src/backend/executor/nodeWindowAgg.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)