fix broken Metaflow Decorators link on GSoC 2026 page

[Gaalbu]

Summary

The GSoC 2026 project ideas page links to https://docs.metaflow.org/api/decorators for "Metaflow Decorators", which returns a 404 since the docs site reorganized this content under a new path.

This updates the link to the current, live page: https://docs.metaflow.org/api/step-decorators — the same URL already used for the "Metaflow Step Decorators" link elsewhere on this same page (in the Nomad extension project section).

Fixes #172

Verification

• Confirmed https://docs.metaflow.org/api/decorators returns HTTP 404.
• Confirmed https://docs.metaflow.org/api/step-decorators resolves and renders the Step Decorators documentation page.
• Ran npm run lint on the changed file (0 errors, only pre-existing line-length warnings unrelated to this change).
• Ran npx docusaurus build to confirm the site still builds cleanly.

[vercel]

@Gaalbu is attempting to deploy a commit to the Metaflow Team on Vercel.

A member of the Team first needs to authorize it.