Merge pull request #7407 from nextcloud/backport/7404/stable20

[stable20] Fix highlighting (missing php start tag)

Author: CarlSchwan

developer_manual/digging_deeper/dashboard.rst

@@ -129,6 +129,7 @@ need to support older versions where the new app boostrap flow is not available,
 however this method is deprecated and will be removed once Nextcloud 19 is EOL.
 
 .. code-block:: php
+    <?php
 
     use OCP\Dashboard\RegisterWidgetEvent;
     use OCP\EventDispatcher\IEventDispatcher;