server: add data: [DONE] to /chat/completions stream response

examples/server/server.cpp

@@ -2986,6 +2986,7 @@ int main(int argc, char ** argv) {
                 }, [&](json error_data) {
                     server_sent_event(sink, "error", error_data);
                 });
+                sink.write("data: [DONE]\n\n", 14);
                 sink.done();
                 return true;
             };