Update SI affix list in SFD_mathformatting.rst

@lamagnifica @catong @WatsonEmily Please review.

(DOC-1107: https://openedx.atlassian.net/browse/DOC-1107)
If the lead-in to this table is accurate, and you can "use the following International System of Units (SI) affixes" I think that what this table is still missing is an example of entering a value using each "affix". Or, maybe instead of "use" in the lead-in, say "represent"? I am really feeling the pain of the guy who wrote to Mark...  I *think* that if a student wants to represent 1000, s/he can enter ``1e3`` (I'm guessing that for 2000 enter ``2e3``, for 3000 ``3e3``, etc.). The standard prefix for 1000 is kilo, and "k" is its symbol, but "kilo" isn't actually what the student is trying to enter, and we're not showing any input *or* output that includes "k" or "kilo". This is wicked confusing!