fix

Author: chenjiahan

README.md

@@ -92,7 +92,7 @@ While it's recommended to run `@rspack/dev-server` via the CLI, you may also cho
 ```js
 import { RspackDevServer } from "@rspack/dev-server";
 import rspack from "@rspack/core";
-import rspackConfig from "./rspack.config.js";
+import rspackConfig from "./rspack.config.mjs";
 
 const compiler = rspack(rspackConfig);
 const devServerOptions = {