infra: specify what kinds of clients in PR template

Author: laurenyu

.github/PULL_REQUEST_TEMPLATE.md

@@ -12,7 +12,7 @@ _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 passed the region in to any/all clients that I've initialized as part of this change.
+- [ ] I have passed the region in to all S3 and STS clients that I've initialized as part of this change.
 - [ ] I have updated any necessary documentation, including [READMEs](https://github.com/aws/sagemaker-python-sdk/blob/master/README.rst) and [API docs](https://github.com/aws/sagemaker-python-sdk/tree/master/doc) (if appropriate)
 
 #### Tests