chore(docs): update credentials.md app router route.js filename (#13025)

Update credentials.md

Author: vinaytz

docs/docs/providers/credentials.md

@@ -37,7 +37,7 @@ If you return an object it will be persisted to the JSON Web Token and the user
 
 The Credentials provider's `authorize()` method also provides the request object as the second parameter (see example below).
 
-```js title="pages/api/auth/[...nextauth].js"
+```js title="pages/api/auth/[...nextauth]/route.js"
 import CredentialsProvider from "next-auth/providers/credentials";
 ...
 providers: [