Updated PR template for new CHANGES file name.

Author: ricellis

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,7 +2,7 @@ Thanks for your hard work, please ensure all items are complete before opening.
 
 - [ ] Tick to sign-off your agreement to the [Developer Certificate of Origin (DCO) 1.1](https://github.com/cloudant/python-cloudant/blob/master/DCO1.1.txt)
 - [ ] You have added tests for any code changes
-- [ ] You have updated the [CHANGES.rst](https://github.com/cloudant/python-cloudant/blob/master/CHANGES.rst) 
+- [ ] You have updated the [CHANGES.md](https://github.com/cloudant/python-cloudant/blob/master/CHANGES.md)
 - [ ] You have completed the PR template below:
 
 ## What