update PR template

Author: Phil Varner

.github/pull_request_template.md

@@ -10,4 +10,4 @@
 - [ ] Code is formatted and linted (run `pre-commit run --all-files`)
 - [ ] Tests pass (run `make test`)
 - [ ] Documentation has been updated to reflect changes, if applicable, and docs build successfully (run `make docs`)
-- [ ] Changes are added to the [CHANGELOG](https://github.com/stac-utils/pystac/blob/master/CHANGES.md), if applicable
+- [ ] Changes are added to the changelog