Correct link in pull request template

Description

The link in the pull request template currently goes to https://github.com/squizlabs/PHP_CodeSniffer/compare/.github/CONTRIBUTING.md while creating a pull request, and https://github.com/squizlabs/PHP_CodeSniffer/pull/.github/CONTRIBUTING.md (which redirects to the first) afterwards. We could change the link to ../blob/HEAD/CONTRIBUTING.md (or ../blob/master/CONTRIBUTING.md) if we want to keep it relative; feel free to adjust this pull request if that's preferred.

Note too that the URL will need to be updated if https://github.com/squizlabs/PHP_CodeSniffer/pull/3830 gets merged before this, or as part of https://github.com/squizlabs/PHP_CodeSniffer/pull/3830 if this gets merged before that.

None required

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
- [ ] This change is only breaking for integrators, not for external standards or end-users.
[x] Documentation improvement

[ ] I have checked there is no other PR open for the same change.
[x] I have read the Contribution Guidelines.
[x] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
[ ] I have added tests to cover my changes.
[ ] I have verified that the code complies with the projects coding standards.
[ ] [Required for new sniffs] I have added XML documentation for the sniff.
[ ] [Required for new files] I have added any new files to the package.xml file.