replace T::BlockNumber with BlockNumberFor<T>

[GitNiko]

It will cause error because associated type BlockNumber not found for T

[netlify[bot]]

✅ Deploy Preview for substrate-docs ready!

Name	Link
🔨 Latest commit	2332529b7fabd96bd6c5c45bfa700ae08ba4fd5f
🔍 Latest deploy log	https://app.netlify.com/sites/substrate-docs/deploys/65025f50064da60007793fc5
😎 Deploy Preview	https://deploy-preview-2046--substrate-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.