Use correct spelling of statistics kind

Tomas Vondra <tomas.vondra@postgresql.org>

Commit: a5f002ad9a2ddb501148a12281efbaacec6f6397
Author: Tomas Vondra <tomas.vondra@postgresql.org>
Date: 2021-03-23T04:01:35Z
Releases: 14.0
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