Update PR template to match other core repos (purescript#254)

Author: JordanMartinez

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,7 +6,7 @@ Clearly and concisely describe the purpose of the pull request. If this PR relat
 
 **Checklist:**
 
-- [ ] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
+- [ ] Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
 - [ ] Linked any existing issues or proposals that this pull request should close
-- [ ] Updated or added relevant documentation in the README and/or documentation directory
+- [ ] Updated or added relevant documentation
 - [ ] Added a test for the contribution (if applicable)