doc: fix POST status code (#1478)

Author: verdie-g

docs/usage/writing/creating.md

@@ -17,7 +17,7 @@ POST /articles HTTP/1.1
 ```
 
 When using client-generated IDs and only attributes from the request have changed, the server returns `204 No Content`.
-Otherwise, the server returns `200 OK`, along with the updated resource and its newly assigned ID.
+Otherwise, the server returns `201 Created`, along with the updated resource and its newly assigned ID.
 
 In both cases, a `Location` header is returned that contains the URL to the new resource.