Changed the spelling of 'dificulty' to 'difficulty' in lib/feature.rb

fixes CNVS-14763

test plan:

- Navigate to your profile page
- Click the down arrow on "Use High Contrast Styles"
- Confirm the spelling of 'difficulty' is now correct and no longer 'dificulty'.

Change-Id: Ife3bf310b5dc45fa609dfaa362814631f2e9e6f4
Reviewed-on: https://gerrit.instructure.com/39854
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>