docs(assertions): update link to migration guide (aws#25602)

The link to the `@aws-cdk/assert` -> `aws-cdk-lib.assertions` migration guide was incorrect, and sent you to a 404 page. The migration guide only exists on the `v1-main` branch now, since the `@aws-cdk/assert` library is not published with v2.


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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*

Author: madeline-k

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

@@ -2,7 +2,7 @@
 
 
 If you're migrating from the old `assert` library, the migration guide can be found in
-[our GitHub repository](https://github.com/aws/aws-cdk/blob/main/packages/@aws-cdk/assertions/MIGRATING.md).
+[our GitHub repository](https://github.com/aws/aws-cdk/blob/v1-main/packages/@aws-cdk/assertions/MIGRATING.md).
 
 Functions for writing test asserting against CDK applications, with focus on CloudFormation templates.