fix: broken emoji key link in contributing.md

[NaufalK25]

Description

Summary

Fixes a broken link to the emoji key in contributing.md.

Changes

• Updated the emoji key link from https://allcontributors.org/docs/en/emoji-key to https://allcontributors.org/en/reference/emoji-key

Why

The old URL no longer resolves (docs site restructured its paths). Contributors following the link were hitting a dead page.

Requirements / Checklist

• Read the Contributing Guidelines
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan.
  Issue number where discussion took place: #xxx
• If this contains a font/glyph add its origin as background info below (e.g. URL)
• Verified the license of any newly added font, glyph, or glyph set. License is: xxx

What does this Pull Request (PR) do?

Fixes a broken link to the emoji key in contributing.md.

• Updated the emoji key link from https://allcontributors.org/docs/en/emoji-key to https://allcontributors.org/en/reference/emoji-key

How should this be manually tested?

1. Open contributing.md
2. Click the emoji key link
3. Confirm it resolves to https://allcontributors.org/en/reference/emoji-key and loads the emoji key page (no 404)

Any background context you can provide?

What are the relevant tickets (if any)?

Screenshots (if appropriate or helpful)