Added callback to code snippet example

docs/sending/start-submission.mdx

@@ -55,6 +55,10 @@ $ curl -L -X POST "$SUBMISSION_API/v1/submissions" \
     \"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\"
     }
 }"
 ```