pgsql: Fix typos in documentation and for one wait event

Fix typos in documentation and for one wait event

These have been found while cross-checking for the use of unique words
in the documentation, and a wait event was not getting generated in a way
consistent to what the documentation provided.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/9b5a3a85-899a-ae62-dbab-1e7943aa5ab1@gmail.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3607dd39ba3d31d30696acacaf76d4a97dbd842f

Modified Files
--------------
doc/src/sgml/func.sgml          | 2 +-
doc/src/sgml/libpq.sgml         | 2 +-
doc/src/sgml/ref/pg_dump.sgml   | 2 +-
src/backend/postmaster/pgstat.c | 2 +-
4 files changed, 4 insertions(+), 4 deletions(-)