[TASK] Fix broken validation translation form docs

The TranslationService uses the additional key 'error' in its translations
of the validation errors.

See https://github.com/TYPO3/TYPO3.CMS/blob/master/typo3/sysext/form/Classes/Service/TranslationService.php#L428.

Releases: master,8.7
Resolves: #82264
Change-Id: Id00b60de0bf1b2b882f583f538be9bc8e40c010f
Reviewed-on: https://review.typo3.org/53851


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>