Fix: add missing Middleware import in middleware-auth docs (#2490)

Author: dipras

docs/openapi-fetch/middleware-auth.md

@@ -13,7 +13,7 @@ Each middleware can provide `onRequest()`, `onResponse()` and `onError` callback
 ::: code-group
 
 ```ts [src/my-project.ts]
-import createClient from "openapi-fetch";
+import createClient, { Middleware } from "openapi-fetch";
 import type { paths } from "./my-openapi-3-schema"; // generated by openapi-typescript
 
 const myMiddleware: Middleware = {