change paging code to fix some bug

[mydaoyuan]

eg

1. when selection on page middle，write some world on full page , then trigger paging , selection after all paragraph will into next page.
2. when editor have two pages, i delete some world on first page, but the second page paragraph dont rise to first page

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/usunil0/slate-paged-demo/3r2T5KRUzcu5i5C2j3tBCsbNPGm1
✅ Preview: https://slate-paged-demo-git-fork-mydaoyuan-fix-paging-usunil0.vercel.app

[mydaoyuan]

and thanks so much to this repo 🎊 , i learn so much