pgsql: Improve some comment wording and grammar in extension.c

Improve some comment wording and grammar in extension.c

Noted while looking at reports of grammatical errors.

Reported-by: albert tan <alterttan1223@gmail.com>
Reported-by: Yuan Li(carol) <carol.li2025@outlook.com>
Discussion: https://postgr.es/m/CAEzortnJB7aue6miGT_xU2KLb3okoKgkBe4EzJ6yJ%3DY8LMB7gw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/94a24b4ee5aa42a80a1aacc8eec1fc6a6938894f

Modified Files
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src/backend/commands/extension.c | 26 +++++++++++---------------
1 file changed, 11 insertions(+), 15 deletions(-)