[DOCS] Correct wrong code sample in CheckboxViewHelper sample

Add multiple="1" to fluid VH.

Releases: main, 12.4, 11.5
Resolves: #101029
Change-Id: I3e9b70e39c67dc776cb4dc3ff57c2c544565e3a1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79312


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>