Use correct spelling of statistics kind

Tomas Vondra <tomas.vondra@postgresql.org>

Commit: 1c457b8b5194bb838b61b02c883ddac4a212ea7e
Author: Tomas Vondra <tomas.vondra@postgresql.org>
Date: 2021-03-23T03:57:45Z
Releases: 12.7
Use correct spelling of statistics kind

A couple error messages and comments used 'statistic kind', not the
correct 'statistics kind'. Fix and backpatch all the way back to 10,
where extended statistics were introduced.

Backpatch-through: 10

Files

Documentation touched