[BUGFIX] Invalid reference to further instructions

[BUGFIX] Invalid reference to further instructions

EXT:lowlevel contains a label suggesting the user to read file
HOWTO_clean_up_TYPO3_installations.txt for more details. This file
has been renamed since TYPO3 7 LTS and both names should be included
in the label since it will be used and translated for older releases
as well.

Resolves: #68572
Releases: master
Change-Id: I9ddebd6903453283afd254a53973e863fc1da41d
Reviewed-on: http://review.typo3.org/42009
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>