pgsql: Fix typos and grammar in code comments

Fix typos and grammar in code comments

Several mistakes have piled in the code comments over the time,
including incorrect grammar, function names and simple typos.  This
commit takes care of a portion of these.

No backpatch is done as this is only cosmetic.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20210924215827.GS831@telsasoft.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e767ddcd354b51fc4c12d6b02e268861bd871fbc

Modified Files
--------------
contrib/tablefunc/tablefunc.c            |  2 +-
src/backend/commands/statscmds.c         | 12 ++++++------
src/backend/executor/nodeTableFuncscan.c | 18 +++++++++---------
src/backend/optimizer/util/pathnode.c    |  2 +-
src/backend/statistics/README            |  6 +++---
src/backend/statistics/README.mcv        |  8 ++++----
src/backend/statistics/extended_stats.c  |  8 ++++----
src/common/pg_lzcompress.c               |  2 +-
8 files changed, 29 insertions(+), 29 deletions(-)