[BUGFIX] Fix typo in form documentation

Releases: master,8.7
Resolves: #85589
Change-Id: I965f559b0307bd477e8f64340a12513a9827842d
Reviewed-on: https://review.typo3.org/57624


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>