Fix tags field for myst_parser

[ousttrue]

Description

Markdown post with tags field in front-matter has format mismatch.

---
tags: ["a", "b"]
---

Interpreted as ["a" and "b"].

I'm fix it.

This is caused in
https://github.com/executablebooks/MyST-Parser/blob/885651fc4e25dbac414c20cfe8fe3232ba4e833b/myst_parser/docutils_renderer.py#L823

And metadata['tags'] store ["a", "b"] text.

Thank you.

[nabobalis]

Thanks.