Use correct spelling of statistics kind

Tomas Vondra <tomas.vondra@postgresql.org>

Commit: fb742ce2687a9fdb103479950603cfbecbed285e
Author: Tomas Vondra <tomas.vondra@postgresql.org>
Date: 2021-03-23T03:54:34Z
Releases: 10.17
Use correct spelling of statistics kind

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

Backpatch-through: 10

Files

Documentation touched