docs: fix typos in 08-mutating-data.mdx

[JamBalaya56562]

Summary

I update the 08-mutating-data.mdx at below points.

1. Delete unnecessary async word
2. Fix some code block extension
3. Add the missing brackets

Improving Documentation

• 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

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 49ed738

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[huozhi]

Thanks!