Fix WebXRInterface code sample in the class reference

This also fixes the code sample's indentation to look correct in the
editor help.

modules/webxr/doc_classes/WebXRInterface.xml

@@ -10,6 +10,8 @@
 		Since WebXR is based on Javascript, it makes extensive use of callbacks, which means that [WebXRInterface] is forced to use signals, where other AR/VR interfaces would instead use functions that return a result immediately. This makes [WebXRInterface] quite a bit more complicated to intialize than other AR/VR interfaces.
 		Here's the minimum code required to start an immersive VR session:
 		[codeblock]
+		extends Node3D
+
 		var webxr_interface
 		var vr_supported = false