Simplify wording of solving_quadratics_by_factoring.*

See: https://docs.google.com/a/khanacademy.org/document/d/1h_0ckPG4qaEbdBk8vWu5AoWjc_1i7G3_O9U6-1ngddk/edit#heading=h.hsxmn3sc3c2f

Auditors: alpert
Merge branch 'master' of github.com:Khan/khan-exercises