Fix typo in MDX documentation (#67432)
I was reading the documentation related to MDX and noticed a typo in
some articles, so I fixed them.
Although I read the [Contribution
Guidelines](https://github.com/vercel/next.js/blob/canary/contributing.md)
this is my first contribution so feel free to comment if I can do
better.
Co-authored-by: Jiwon Choi <devjiwonchoi@gmail.com>
