[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>

typo3/sysext/core/Documentation/Changelog/master/Feature-90333-Dashboard.rst

@@ -60,6 +60,7 @@ Tag you widget in :file:`EXT:your_extension/Configuration/Services.yaml`:
 
       # Variant 1, widget identifier as attribute
       Vendor\Extension\Widgets\MyFirstWidget:
+        arguments: ['widget-identifier-1']
         tags:
           - name: dashboard.widget
             identifier: widget-identifier-1
@@ -69,6 +70,7 @@ Tag you widget in :file:`EXT:your_extension/Configuration/Services.yaml`:
       # to share the same class
       widget.identifier:
         class: Vendor\Extension\Widgets\MySecondWidget
+        arguments: ['widget-identifier-1']
         tags:
           - name: dashboard.widget
             # If ommited, the identifier would be the service name, thus 'widget.identifier'