Add group and specification IDs to letter event example

Author: m-houston

internal/events/schemas/examples/letter.DISPATCHED.json

@@ -1,10 +1,12 @@
 {
   "data": {
     "domainId": "f47ac10b-58cc-4372-a567-0e02b2c3d479",
+    "groupId": "client_template",
     "origin": {
       "source": "/data-plane/letter-rendering/prod/render-pdf",
       "subject": "origin/00f3b388-bbe9-41c9-9e76-052d37ee8988/letter-rendering/letter-request/0o5Fs0EELR0fUjHjbCnEtdUwQe4_0o5Fs0EELR0fUjHjbCnEtdUwQe5"
     },
+    "specificationId": "1y3q9v1zzzz",
     "status": "DISPATCHED"
   },
   "datacontenttype": "application/json",