chore: fix code snippet in Readme (#34450)

### Issue # (if applicable)
N/A

### 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: moelasmar

packages/aws-cdk-lib/cx-api/README.md

@@ -731,4 +731,4 @@ _cdk.json_
     "@aws-cdk/aws-s3:publicAccessBlockedByDefault": true
   }
 }
-'''
+```