docs: rename `md` to `mdx` in `errors` directory (#71785)
## Description
Some files use `md` extension.
So, I corrected it by changing the extension to `mdx`.
### Improving Documentation
- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR.
- [x] Read the Docs Contribution Guide to ensure your contribution
follows the docs guidelines:
https://nextjs.org/docs/community/contribution-guide
Co-authored-by: JJ Kasper <jj@jjsweb.site>
