add code snippet to clarify how to set location constraint directly

[OswaldHe]

aws-doc-sdk-examples Pull Request

Thank you for making a submission to the aws-doc-sdk-examples repository. For more information about submitting pull requests to this repository, see Guidelines for contributing.

NOTE: This PR template contains three sections. Depending on the reason for your pull request, please fill out the section that applies to you and then remove the other two sections.

Existing Example Update

The submitter has:

• [x] Confirmed that the correct copyright is included in all files.
• [ ] Major code changes have been reviewed, and the submitter has incorporated review comments.
• [ ] Changed or added comments and strings have been reviewed, and the submitter has incorporated any and all suggested edits.

Description of Changes

Please describe the changes you have made here: The original code example doesn't clearly indicate the way to set the bucket location constraint in create_bucket.cpp. Though there is one in the unit test, it uses a different way to create the bucket, and normally developer will not look at the unit test for a quick reference.

• [x] I have tested my changes and created unit tests for new code paths.
• [ ] Changes have been reviewed, and all reviewer comments have been incorporated.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.