fix: typo into 03-css-in-js.mdx page (#51761)
### Improving Documentation
Just fixing a typo into "alphabetical" word
- Run `pnpm prettier-fix` to fix formatting issues before opening the PR.
- Read the Docs Contribution Guide to ensure your contribution follows the docs guidelines: https://nextjs.org/docs/community/contribution-guide
### What?
Misleading typo
