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_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/75459bc427d1598d72f9663ffa0fb910844d54a9

Modified Files
--------------
contrib/bloom/blscan.c                    | 2 +-
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 +-
13 files changed, 15 insertions(+), 15 deletions(-)