bump a translation key on course homepage

we changed the name of the interpolation argument without updating the key, so
translations that weren't updated in time are broken.

fixes CNVS-14193

test plan:
- go to the course homepage in a language that does not have an updated
  translation for this key
- it should not error

Change-Id: Ideeceacdc6fcf3178c4e6bc94844b9cf3d2004b3
Reviewed-on: https://gerrit.instructure.com/37604
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>