Fix contribution guidelines link in PR templates

[iSame7]

This PR fix the path for contribution guidelines in PR templates.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

Thanks, @iSame7!

[mdznr]

The relative references are correct with respect to a local filesystem, but don’t work when viewing a pull request on github.com.

If you're on a page like #16 and click on a link to ../CONTRIBUTING.md, it goes to https://github.com/apple/swift-algorithms/CONTRIBUTING.md which gives a 404. The correct link would be https://github.com/apple/swift-algorithms/blob/main/CONTRIBUTING.md

[LemonSpike]

@mdznr I don't think we need to keep the link correct for the file viewer, correct? This relates to #134.