Improve GitHub PR template (withastro#902)

* Add a prompt for issue # in PR template

* Switch from checklist to delete in “what kind of changes”

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,16 +1,17 @@
 <!-- Thank you for opening a PR! We really appreciate you taking the time to help out 🙌 -->
 
 #### What kind of changes does this PR include?
-<!-- Place an X in the [ ] for any of these that apply -->
+<!-- Delete any that don’t apply -->
 
-- [ ] Minor content fixes (broken links, typos, etc.)
-- [ ] New or updated content
-- [ ] Translated content
-- [ ] Changes to the docs site code
-- [ ] Something else!
+- Minor content fixes (broken links, typos, etc.)
+- New or updated content
+- Translated content
+- Changes to the docs site code
+- Something else!
 
 #### Description
 
+- Closes # <!-- Add an issue number if this PR will close it. -->
 - What does this PR change? Give us a brief description.
 - Did you change something visual? A before/after screenshot can be helpful.