pgsql: Clarify use of "statistics objects" in the code

Clarify use of "statistics objects" in the code

The code inconsistently used "statistic object" or "statistics" where
the correct term, as discussed, is actually "statistics object". This
improves the state of the code to be more consistent.

While on it, fix an incorrect error message introduced in a4d75c8. This
error should never happen, as the code states, but it would be
misleading.

Author: Justin Pryzby
Reviewed-by: Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/20210924215827.GS831@telsasoft.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/2cf9cf5d7b5e66202389618a4c08945da352b35c

Modified Files
--------------
src/backend/commands/statscmds.c | 28 ++++++++++++++--------------
src/backend/commands/tablecmds.c | 8 ++++----
src/backend/parser/parse_utilcmd.c | 6 +++---
src/backend/statistics/extended_stats.c | 18 +++++++++---------
src/backend/utils/adt/selfuncs.c | 28 ++++++++++++++--------------
5 files changed, 44 insertions(+), 44 deletions(-)