Correct default value for new XRMode field

[diarmidmackenzie]

Description:

1.5.0 docs say the default value for the new field is vr, ar, xr. In fact the default value is vr, and ar & xr are other possible values.

The relevant line of the schema is:

XRMode: {default: 'vr', oneOf: ['vr', 'ar', 'xr']}

Changes proposed:

Update docs to indicate the actual default value.

[dmarcos]

Thanks!