Use correct spelling of statistics kind

Tomas Vondra <tomas.vondra@postgresql.org>

Commit: 16568c0a8aee85052cfd3e465d21d1e1d1b442e1
Author: Tomas Vondra <tomas.vondra@postgresql.org>
Date: 2021-03-23T03:56:17Z
Releases: 11.12
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