docs(custom-resource): correcting links to S3File example code (aws#1…

…6822)

Was reading the custom-resource documentation this morning (https://docs.aws.amazon.com/cdk/api/latest/docs/custom-resources-readme.html#s3file) and realized that both the S3File and S3Assert sections linked to the S3Assert sample code.

Fixed the S3File links to point to the right files, and verified that they looked correct in the rendered Markdown. 

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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/custom-resources/README.md

@@ -301,8 +301,8 @@ 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-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).
+[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).
 
 The following example will create the file `folder/file1.txt` inside `myBucket`
 with the contents `hello!`.