[fix] Add flexform to extbase plugin example

Author: thomasrawiel

Documentation/ApiOverview/FlexForms/_codesnippets/_extbase_plugin.php

@@ -10,7 +10,6 @@
     'my-extension-icon',
     'plugins',
     'Plugin description',
-    'FILE:EXT:my_extension/Configuration/FlexForms/MyFlexform.xml',
 );
 
 ExtensionManagementUtility::addToAllTCAtypes(
@@ -19,3 +18,9 @@
     $ctypeKey,
     'after:subheader',
 );
+
+ExtensionManagementUtility::addPiFlexFormValue(
+    '',
+    'FILE:EXT:myext/Configuration/FlexForms/MyFlexform.xml',
+    $ctypeKey,
+);