[fix] Add flexform to extbase plugin example

[thomasrawiel]

This PR fixes the example on how to add a Flexform to an extbase plugin.

There is no parameter for the flexform path in ExtensionUtility:registerPlugin(). It always has been

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue(
    $pluginSignature, // or since TYPO3 12 $cTypeKey
    'FILE:EXT:example/Configuration/FlexForms/Registration.xml'
);

Source:
Feature: #82809
11.5 API
12.4 API
13.4 API

[linawolf]

Thank you for the contribution.

This has actually been deprecated with TYPO3 14.0: https://docs.typo3.org/permalink/changelog:deprecation-107047-1751984220, in favour of Feature: #107047 – FlexForm enhancements: Direct plugin registration and raw TCA support (https://docs.typo3.org/permalink/changelog:feature-107047-1751984817).

The example in the documentation was therefore updated intentionally. If you need to look up behaviour for older TYPO3 versions, please use the version switch in the top bar to select the relevant TYPO3 version. Also note the info boxes that indicate when specific features were introduced.

Closing this PR as the current documentation is correct for TYPO3 14+.