docs(mdx): fix package name in .md handling section (#97131)
The MDX guide’s “Handling `.md` files” section referenced `next/mdx`,
but the package used throughout the guide is `@next/mdx`.
This updates the text to match the correct package name to avoid
confusion for readers configuring `.md` support.
Attribution: This change was originally authored by @pavan-sh in #89710.
Co-authored-by: Pavan Shinde <pavann97@gmail.com>
