docs(custom-resources): fix dead links (aws#14989)

Fixes aws#14895


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packages/@aws-cdk/custom-resources/README.md

@@ -268,8 +268,8 @@ This module includes a few examples for custom resource implementations:
 
 Provisions an object in an S3 bucket with textual contents. See the source code
 for the
-[construct](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/custom-resources/test/provider-framework/integration-test-fixtures/s3-file.ts) and
-[handler](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/custom-resources/test/provider-framework/integration-test-fixtures/s3-file-handler/index.ts).
+[construct](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/custom-resources/test/provider-framework/integration-test-fixtures/s3-assert.ts) and
+[handler](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/custom-resources/test/provider-framework/integration-test-fixtures/s3-assert-handler/index.py).
 
 The following example will create the file `folder/file1.txt` inside `myBucket`
 with the contents `hello!`.