fix link to Promise on MDN

[peterbe]

The link for "Promise" goes to the Polish translation on MDN. It should be without language so people get MDN in their desired language instead.

Checklist

• I have read the documentation.
• I have included a pull request description of my changes.
• I have included some tests.
• If it's a new feature, I have included documentation updates in both the README and the types.

[szmarczak]

Thanks! 🙈