OpenXR passthrough documentation refers to OVRMobile plugin

**Your Godot version:**

3.x

**Issue description:**

I don't know the correct code, because I've never done passthrough in Godot 3 with the OpenXR plugin, so I can't fix it (at least without some more research) but I noticed when looking at the docs (because someone linked them on Discord) that the code is loading part of the OVRMobile plugin which can't be right. :-)


This should be loading something from the OpenXR plugin instead.

**URL to the documentation page:**

https://docs.godotengine.org/en/stable/tutorials/vr/openxr/passthrough.html