[TASK] Fix broken identifier in form docs

Within the docs, an incorrect identifier is used for a custom finisher.
This has been fixed and is in line with the example code above.

Releases: master,8.7
Resolves: #82234
Change-Id: I41881f825e7961f4945805360683c3189f2f9fcb
Reviewed-on: https://review.typo3.org/53825


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>