Added callback to Javascript example. (planning#74)

docs/sending/start-submission.mdx

@@ -88,6 +88,10 @@ const data = JSON.stringify({
   "serviceType": {
     "name": "Antrag Name",
     "identifier": "urn:de:fim:leika:leistung:99102013104000"
+  },
+  "callback": {
+    "url": "https://my-onlineservice.example.org/callbacks/fit-connect",
+    "secret": "insecure_unsafe_qHScgrg_kP-R31jHUwp3GkVkGJolvBchz65b74Lzue0"
   }
 });