chore: Fix PR link for PRs

[jsoref]

Fixes

PRs when viewed have funny base URLs, some are fine, and some are not, e.g.:

• https://github.com/sendgrid/sendgrid-java/compare/CONTRIBUTING.md (when viewed from main...jsoref:pr-tmplate)
• https://github.com/sendgrid/sendgrid-java/pull/CONTRIBUTING.md (when viewed from a PR -- e.g. chore: Fix PR link for PRs #651)

-- These URLs don't work, which is problematic if one is trying to follow the link while writing a PR or while looking at an existing PR.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

If you have questions, please file a support ticket, or create a GitHub Issue in this repository.

[childish-sambino]

Nice catch!