[DOCS] Put example input in backtics in form manual

This prevents warnings during rendering and also looks better.

Resolves: #103355
Releases: main, 12.4, 11.5
Change-Id: I0a677f4f1c5ee8253538dd414fe23384ad49b6e9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83394


Tested-by: Chris Müller <typo3@krue.ml>
Reviewed-by: Chris Müller <typo3@krue.ml>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Lina Wolf <112@linawolf.de>
Tested-by: Lina Wolf <112@linawolf.de>