Document the necessary step to "Make unique" the Mesh in the Lightmaps Tutorial

**Your Godot version:**
3.2.3, 3.3

**Issue description:**
When baking Lightmaps following the tutorial in the documentation, the Lightmaps will show up in the Editor, but not when running the game. The solution is to "Make unique" the Mesh, but this is unfortunately not documented.

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/tutorials/3d/baked_lightmaps.html#setting-up-meshes