css, m.htmlsanity: use a <div> for .m-figure-description.

css, m.htmlsanity: use a <div> for .m-figure-description.

I can't read, apparently. Thought that the "flow content" on MDN doesn't
allow any block elements, only inline, but it does. So there's no need
to put a stupid <span> there and then try to make a block element out of
it via CSS again.