Fix minor bug in the Sphinx configuration.

[rpgoldman]

Warning revealed that the value of html_theme_path was of the wrong type.

[ColCarroll]

To be clear, this change suppresses the warning, but doesn't change how the docs look, right?

[rpgoldman]

To be clear, this change suppresses the warning, but doesn't change how the docs look, right?

Yes, that's correct. TBH, I don't know how it worked before, because as far as I could tell, sphinx couldn't find that style directory. But it works now.

I think we might be happier with a themes/ subdirectory with semantic-sphinx contained in it, but that probably isn't really necessary unless we start using more than a single theme.