Update PULL_REQUEST_TEMPLATE.md to include issue cross-reference.

This changes the PR template to prompt/remind the user to link the issue it resolves. This is to make sure that it reduces duplicate PRs, as having a back link would make it easier to know that there's work being done on a particular issue.

Author: chahak13

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,6 +3,8 @@
 ## PR Checklist
 
 <!-- Please mark any checkboxes that do not apply to this PR as [N/A]. -->
+**Linked Issue**
+- [ ] Mentioned issue that it closes using a phrase like "closes #123" to link it back to the issue.
 
 **Documentation and Tests**
 - [ ] Has pytest style unit tests (and `pytest` passes)