pgsql: Fix misc typos, mostly in comments.

Fix misc typos, mostly in comments.

A collection of typos I happened to spot while reading code, as well as
grepping for common mistakes.

Backpatch to all supported versions, as applicable, to avoid conflicts
when backporting other commits in the future.

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a9793e07351eb47c926a9e885956bf2d10da66dd

Modified Files
--------------
src/backend/access/transam/multixact.c    | 4 ++--
src/backend/commands/cluster.c            | 2 +-
src/backend/executor/nodeModifyTable.c    | 2 +-
src/backend/replication/logical/origin.c  | 2 +-
src/bin/initdb/t/001_initdb.pl            | 2 +-
src/bin/pg_upgrade/tablespace.c           | 2 +-
src/interfaces/ecpg/ecpglib/execute.c     | 4 ++--
src/pl/tcl/pltcl.c                        | 2 +-
src/test/regress/expected/rowsecurity.out | 2 +-
src/test/regress/expected/rules.out       | 2 +-
src/test/regress/sql/rowsecurity.sql      | 2 +-
src/test/regress/sql/rules.sql            | 2 +-
12 files changed, 14 insertions(+), 14 deletions(-)