fix(.github/config.yml): fix link for contribution guidelines - I1328

[elit-altum]

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

Fixes #1328
Changed the url for the contribution guidelines which was leading to 404 error before in .github/config.yml

[elit-altum]

@catarak Please review!
Thank you!

[andrewn]

Thanks for updating this 😄