Fix typo in regression test comment.

Etsuro Fujita <efujita@postgresql.org>

Commit: 8c88d21ced41b3a8b227235765065158b723e280
Author: Etsuro Fujita <efujita@postgresql.org>
Date: 2019-08-29T09:45:02Z
Releases: 11.6
Fix typo in regression test comment.

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