Update PR template for main branch rename (#613)

Author: lossyrob

.github/pull_request_template.md

@@ -10,4 +10,4 @@
 - [ ] Tests pass (run `scripts/test`)
 - [ ] Documentation has been updated to reflect changes, if applicable
 - [ ] This PR maintains or improves overall codebase code coverage.
-- [ ] Changes are added to the [CHANGELOG](https://github.com/stac-utils/pystac/blob/develop/CHANGELOG.md). See [the docs](https://pystac.readthedocs.io/en/latest/contributing.html#changelog) for information about adding to the changelog.
+- [ ] Changes are added to the [CHANGELOG](https://github.com/stac-utils/pystac/blob/main/CHANGELOG.md). See [the docs](https://pystac.readthedocs.io/en/latest/contributing.html#changelog) for information about adding to the changelog.