Underscores in anchor

Is there a reason the anchor defined in [MyST style guide](https://github.com/canonical/sphinx-docs-starter-pack/blob/main/docs/reference/style-guide-myst.md) uses underscores instead of the minus sign (`myst_style_guide`)? Given that

a) Other anchors in the repo (when defined) use the minus char.
b) Sphinx generates the HTML anchor with minus chars, so it's confusing.

Open docs/reference/style-guide-myst.md from the linked MyST style guide and inspect the myst_style_guide anchor alongside other anchors in the repository. Confirm how Sphinx renders the heading, then make the anchor use a minus sign consistently and verify the generated HTML link target.

Written by the indexing model from the issue text.