Update pull_request_template.md

Author: chernser

.github/pull_request_template.md

@@ -1,9 +1,10 @@
 ## Summary
 <!-- A short description of the changes with a link to an open issue. -->
 
+Closes <!-- Link to an issue to close on merge. -->
 ## Checklist
 Delete items not relevant to your PR:
-- [ ] Closes <!-- Link to an issue to close on merge. -->
+- [ ] Closes #<issue ref>
 - [ ] Unit and integration tests covering the common scenarios were added
 - [ ] A human-readable description of the changes was provided to include in CHANGELOG
 - [ ] For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials