Update SI affix list in SFD_mathformatting.rst

@lamagnifica @catong @WatsonEmily Please review.

(DOC-1107: https://openedx.atlassian.net/browse/DOC-1107)
I built an html version locally, and noticed that these images below are just a little bit differently sized. The formatting would be nicer if the canvas size of each was the same. Possible in your timeframe?