Update PR template to render empty checkboxes

Author: underyx

.github/pull_request_template.md

@@ -4,6 +4,6 @@ A short description about the changes in this pull request. If the pull request
 
 ## Checklist
 
-- [] Tests covering the new functionality have been added
-- [] Documentation has been updated OR the changes are too minor to be documented
-- [] The Changes are listed in the `CHANGELOG.md` OR the changes are insignificant
+- [ ] Tests covering the new functionality have been added
+- [ ] Documentation has been updated OR the changes are too minor to be documented
+- [ ] The Changes are listed in the `CHANGELOG.md` OR the changes are insignificant