Correct rendering of maths

The asterisks work fine with GH's markdown renderer, but on the RPi site the parser produces italics instead, hence I've replaced the asterisks with x's for now (which is also consistent with one of the earlier bits of maths in the prose. It would probably be more correct to use × (unicode multiplication character) which would eliminate ambiguity in case it gets rendered with a serif font, but I'm not sure how the main site's parser copes with UTF-8 yet