pgsql: Fix typos in comments.

Fix typos in comments.

Also adjust the phrasing in the comments.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Author: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Gurjeet Singh <gurjeet@singh.im>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAPmGK17%3DPHSDZ%2B0G6jcj12buyyE1bQQc3sbp1Wxri7tODT-SDw%40mail.gmail.com
Backpatch-through: 15

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c8a393537c39077efa20357fa13d136c643cff5a

Modified Files
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src/backend/utils/activity/pgstat_database.c     | 4 ++--
src/backend/utils/activity/pgstat_function.c     | 4 ++--
src/backend/utils/activity/pgstat_relation.c     | 4 ++--
src/backend/utils/activity/pgstat_subscription.c | 4 ++--
4 files changed, 8 insertions(+), 8 deletions(-)