fix punctuation error in formula quiz directions

closes CNVS-5050

test plan:
  - add a formula question to an existing quiz
  - verify that the sentence does not missue the apostrophe

Change-Id: Ib1d16c77141f7deeac00e63f795bf2edbffeb39b
Reviewed-on: https://gerrit.instructure.com/28437
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>