SETUP.md git cherry-pick bad revision 'cb21841'

The [instructions](https://github.com/GodotVR/godot_arcore/blob/master/SETUP.md#1-compiling-the-engine--editor) for setting up the development environment are wrong:

```
git cherry-pick changes/arcore_changes cb21841
```

Returns:

```
fatal: bad revision 'cb21841'
```

As the commit `cb21841` never existed