Fix incorrect linking of the code of conduct by making the link absolute. This makes the link work via both the main repository pages' Contributing tab as well as when navigating directly to the Contributing file.

🔗 Related issue

Fixes: #54

📚 Type of change

• 📝 Examples / docs / tutorials
• 🐛 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🚨 Security fix
• ⬆️ Dependencies update

✔️ Checklist

• I've read the Code of Conduct document.
• I've read the Contributing guide.