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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/42e2a580713201645d7caa4b27713ac777432d8d

Modified Files
--------------
doc/src/sgml/file-fdw.sgml      | 2 +-
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 +-
5 files changed, 5 insertions(+), 5 deletions(-)