Fix changelog link

[mockdeep]

🎩 What? Why?

This updates the changelog link in the contributing doc to point to the appropriate location. The current one reaches a "Page Not Found" error.

Testing

Visit the link.

📋 Checklist

• ❓ CONSIDER adding a unit test if your PR resolves an issue.
• ✔️ DO check open PR's to avoid duplicates.
• ✔️ DO keep pull requests small so they can be easily reviewed.
• ✔️ DO build locally before pushing.
• ✔️ DO make sure tests pass.
• ✔️ DO make sure any new changes are documented in docs/.
• ✔️ DO add and modify seeds if necessary.
• ✔️ DO add CHANGELOG upgrade notes if required.
• ✔️ DO add to GraphQL API if there are new public fields.
• ✔️ DO add link to MetaDecidim if it's a new feature.
• ❌AVOID breaking the continuous integration build.
• ❌AVOID making significant changes to the overall architecture.

[andreslucena]

Thanks! I changed that URL a couple of weeks ago and forgot to search if we have it linked somewhere. We should probably start working with redirects when making these kinds of changes.