change wording on question bank editing

fixes #CNVS-13065

Per Erin Hallmark, "Question Group" makes
more sense than "Quiz Group"

Test Plan:
 -Login as an instructor
 -try to add a question to a question group
 -see the better label that using the preferred
    wording

Change-Id: I372e4bce7e223557ae5ce3a08f2d91882bd45857
Reviewed-on: https://gerrit.instructure.com/39760
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>