Use given_name instead of first_name in Appendix D mdoc examples

[javereec]

The DCQL examples in appendix D use either first_name or given_name seemingly without much consistency. While I would find it more satisfying to always use given_name, that might be a matter of personal preference. However, when referencing specified mdoc namespaces such as org.iso.18013.5.1, I do think we need to use the defined values (given_name and not first_name). See example below for the incorrect use imo.

{
  "credentials": [
    {
      "id": "my_credential",
      "format": "mso_mdoc",
      "meta": {
        "doctype_value": "org.iso.7367.1.mVRC"
      },
      "claims": [
        {"path": ["org.iso.7367.1", "vehicle_holder"]},
        {"path": ["org.iso.18013.5.1", "first_name"]}
      ]
    }
  ]
}

I'll create a quick PR