Fix incorrect PR number in changelog for 3.2.4

[36degrees]

Summary

Proposed change:

Link to the correct PR (#1437) in the changelog entry for 3.2.4

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• [x] Proposed change helps towards purpose of this project.
• [x] Documentation is added / updated to describe proposed change.
• [x] Tests are added / updated to cover proposed change.
• [x] Changelog has an entry for proposed change (if user-facing fix or feature).

[36degrees]

It would also be good if a release could be created so that it shows in https://github.com/mozilla/nunjucks/releases (and the latest release is reflected in the sidebar of the main repo page).

[36degrees]

Thanks for creating the release, and the speedy merge 🙌🏻