[DOCS] Add missing key for widget registration

Using the example registration of a custom widget would lead
to an ArgumentCountError respectively an RuntimeException as
the `arguments` key is missing. Therefore the key is added to
the configuration examples.

Resolves: #90501
Releases: master
Change-Id: Ib6666169fda610dd6393f4869f476d0a29edd56e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63386


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>