fix(amplify-alpha): example code for adding a custom rule is wrong (#34353)

Closes #34351

### Reason for this change



User reported an issue with the example code in the README.

### Description of changes



Fix the example.

### Describe any new or updated permissions being added


None

### Description of how you validated changes



No code changes. Existing rossetta test are sufficient. 

### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: iliapolo

packages/@aws-cdk/aws-amplify-alpha/README.md

@@ -102,12 +102,14 @@ Auto build and pull request preview are enabled by default.
 Add custom rules for redirection:
 
 ```ts
+import { CustomRule } from '@aws-cdk/aws-amplify-alpha';
+
 declare const amplifyApp: amplify.App;
-amplifyApp.addCustomRule({
+amplifyApp.addCustomRule(new CustomRule({
   source: '/docs/specific-filename.html',
   target: '/documents/different-filename.html',
   status: amplify.RedirectStatus.TEMPORARY_REDIRECT,
-});
+}));
 ```
 
 When working with a single page application (SPA), use the