fix PR template

Author: philvarner

.github/pull_request_template.md

@@ -10,5 +10,4 @@
 **PR Checklist:**
 
 - [ ] This PR has **no** breaking changes.
-- [ ] This PR does not make any changes to the core spec in the `stac-spec` directory (these are included as a subtree and should be updated directly in [radiantearth/stac-spec](https://github.com/radiantearth/stac-spec))
-- [ ] I have added my changes to the [CHANGELOG](https://github.com/radiantearth/stac-api-spec/blob/main/CHANGELOG.md) **or** a CHANGELOG entry is not required.
+- [ ] I have added my changes to the [CHANGELOG](https://github.com/stac-api-extensions/children/blob/main/CHANGELOG.md) **or** a CHANGELOG entry is not required.