fix 'helpful' en-dashing in headings

[nedbat]

The headings get double-hyphens turned into en-dashes:

I'm not sure if this fixes it (I haven't used .md in Sphinx before), but it might?

[nedbat]

Helpfully, readthedocs built the pull request. Unhelpfully, the en-dashes are still there :(

[nedbat]

With backticks, the headers are formatted as code, which arguably is more correct, but also a little odd in a header:

[simonw]

https://shot-scraper--81.org.readthedocs.build/en/81/ shows a preview.

https://shot-scraper--81.org.readthedocs.build/en/81/installation.html#shot-scraper-shot-help

Compared to https://shot-scraper.datasette.io/en/stable/installation.html#shot-scraper-shot-help which exhibits the bug:

[simonw]

Thanks for this!