Markdownify the checkboxes in the PR template (#6607)

In order to be rendered as checkboxes by Github, the square brackets
need a space between them.
Co-authored-by: NTaylor Vesely <tvesely@pivotal.io>
Co-authored-by: NBen Christel <bchristel@pivotal.io>

PULL_REQUEST_TEMPLATE.md

 ## Here are some reminders before you submit the pull request
-- [] Add tests for the change
-- [] Document changes
-- [] Communicate in the mailing list if needed
-- [] Pass `make installcheck`
+- [ ] Add tests for the change
+- [ ] Document changes
+- [ ] Communicate in the mailing list if needed
+- [ ] Pass `make installcheck`
\ No newline at end of file