add another item

Author: laurenyu

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,6 +10,7 @@ _Put an `x` in the boxes that apply. You can also fill these out after creating
 
 - [ ] I have read the [CONTRIBUTING](https://github.com/aws/sagemaker-python-sdk/blob/master/CONTRIBUTING.md) doc
 - [ ] I used the commit message format described in [CONTRIBUTING](https://github.com/aws/sagemaker-python-sdk/blob/master/CONTRIBUTING.md#committing-your-change)
+- [ ] I have used the regional endpoint when creating S3 and/or STS clients (if appropriate)
 - [ ] I have added tests that prove my fix is effective or that my feature works (if appropriate)
 - [ ] I have checked that my tests are not configured for a specific region or account (if appropriate)
 - [ ] I have updated any necessary [documentation](https://github.com/aws/sagemaker-python-sdk/blob/master/README.rst) (if appropriate)