[BUGFIX] Fix typo in sysfolder related doktype check

The changes in #18079 have a small, but nasty typo, that makes it
impossible to correctly check if the page is a sysfolder.

Resolves: #90975
Related: #18079
Releases: master
Change-Id: Iec8d119a9d29adec742f322a66102ce77a6eb16a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64095


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>