docs: correct the addToRolePolicy example (#23014)

This one threw me for a bit as a newbie - using the example as provided resulted in an error:

```
Syntax errors in policy. (Service: AmazonIdentityManagement; Status Code: 400; Error Code: MalformedPolicyDocument; Request ID: <redacted>; Proxy: null)
```

A bit of sleuthing led me to this as the correct format.

----

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Author: jak

packages/@aws-cdk/integ-tests/lib/assertions/providers/provider.ts

@@ -260,8 +260,8 @@ export class AssertionsProvider extends Construct {
    * declare const provider: AssertionsProvider;
    * provider.addToRolePolicy({
    *   Effect: 'Allow',
-   *   Action: 's3:GetObject',
-   *   Resources: '*',
+   *   Action: ['s3:GetObject'],
+   *   Resource: ['*'],
    * });
    */
   public addToRolePolicy(statement: any): void {