docs: Updated reference json file for verification payload to latest …

…reqs

In reference payload for the verification request:
* Added required credentialConfigurationId fiel, hardcoded to example
  id.
* Removed empty Id field that caused serde to panic.

agent_api_rest/res/open-badge-request.json

@@ -1,8 +1,8 @@
 {
   "offerId": "c86289fa-b105-4ec3-9326-a02436788f11",
+  "credentialConfigurationId": "w3c_vc_credential",
   "credential": {
     "credentialSubject": {
-      "id": "",
       "type": ["AchievementSubject"],
       "achievement": {
         "id": "https://example.com/achievements/21st-century-skills/teamwork",