Update SI affix list in SFD_mathformatting.rst

@lamagnifica @catong @WatsonEmily Please review.

(DOC-1107: https://openedx.atlassian.net/browse/DOC-1107)
line 36 above, "curved parentheses" --> show the characters expected? "Only use parentheses, that is, the ( ) characters. Don't use the bracket [ ] or brace { } characters."