pgsql: Fix typos and grammar in documentation and code comments

Fix typos and grammar in documentation and code comments

Comment fixes are applied on HEAD, and documentation improvements are
applied on back-branches where needed.

Author: Justin Pryzby
Discussion: /messages/by-id/20210408164008.GJ6592@telsasoft.com
Backpatch-through: 9.6

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dc6d285c2ef997f3a8d4c9f0e0599b62b0cbd054

Modified Files
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doc/src/sgml/maintenance.sgml | 2 +-
doc/src/sgml/ref/create_table.sgml | 4 ++--
doc/src/sgml/ref/createuser.sgml | 2 +-
3 files changed, 4 insertions(+), 4 deletions(-)