chore(doc): fix a broken link in the README.md (#30789)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

Just fix a broken link.

### Description of changes



### Description of how you validated changes



### 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/core/README.md

@@ -268,7 +268,7 @@ In order to mimic strong references, a Custom Resource is also created in the co
 stack which marks the SSM parameters as being "imported". When a parameter has been successfully
 imported, the producing stack cannot update the value.
 
-See the [adr](https://github.com/aws/aws-cdk/blob/main/packages/@aws-cdk/core/adr/cross-region-stack-references)
+See the [adr](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/core/adr/cross-region-stack-references.md)
 for more details on this feature.
 
 ### Removing automatic cross-stack references