[FOLLOW-UP] Rename rst file

Add missing file extension to rst file.

Resolves: #58929
Releases: master
Change-Id: I39e9286914eb92db70dfd68bcd837b9fe0183f2f
Reviewed-on: http://review.typo3.org/35647
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>