doc: add missing LoadingState.Streaming (#37903) (#37908)

The final query is missing this and does not work without it.

(cherry picked from commit 5c69f899b5)

Co-authored-by: Teppo Kurki <teppo.kurki@iki.fi>

docs/sources/developers/plugins/build-a-streaming-data-source-plugin.md

@@ -120,6 +120,7 @@ query(options: DataQueryRequest<MyQuery>): Observable<DataQueryResponse> {
         subscriber.next({
           data: [frame],
           key: query.refId,
+          state: LoadingState.Streaming,
         });
       }, 100);