Update PULL_REQUEST_TEMPLATE.md

[TeaganKing]

Description of changes: Update contributor's guide link in PR template-- thanks Alice for pointing this out!

All PRs Checklist:

• Have you followed the guidelines in our Contributor's Guide?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you made sure that the pre-commit checks passed (#8 in Adding Notebooks Guide)?
• Have you successfully tested your changes locally?

[mnlevy1981]

LGTM!