Correct link in pull request template

[fredden]

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 #3830 gets merged before this, or as part of #3830 if this gets merged before that.

Suggested changelog entry

None required

Types of changes

• 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.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• 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.

[jrfnl]

I'd adjusted this template at the same time as creating/updating & moving the CONTRIBUTING doc, so I took a leap of faith that PR #3830 would/will be merged as well. Once that is merged, the link should work correctly ;-)

(and if it doesn't, let's update it after PR #3830 to prevent having to do yet another follow-up PR)

[fredden]

Superseded by PHPCSStandards/PHP_CodeSniffer#40 / PHPCSStandards/PHP_CodeSniffer@4caa4ef