chore(secretmanager): adding missed header in README (#32532)

This PR is to fix the readme, and add back a header that got deleted by mistake in this PR https://github.com/aws/aws-cdk/pull/26329/files#diff-c70c65923cf81d0d8bdd04d2b6160ec67aa5556be569ac08f44359df53de8bb9

### 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)

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

packages/aws-cdk-lib/aws-secretsmanager/README.md

@@ -93,6 +93,8 @@ const secret = new secretsmanager.Secret(this, 'Secret', { encryptionKey: key })
 secret.grantRead(otherAccount);
 ```
 
+## Rotating a Secret
+
 ### Using a Custom Lambda Function
 
 A rotation schedule can be added to a Secret using a custom Lambda function: