en: Correct em dash and others

ch-01_en.md: There should be no spaces around em dash as Shelly
suggested. More footnotes are added from OSU.
URL: http://oregonstate.edu/instruct/phl302/texts/wollstonecraft/woman-a.html

intro_en.md: Correct em dash.

sketch_en.md: Correct em dash.
en: Add italic markdown tag

sketch_en.md: Italic markdown tag is added for "Political Justice"