fix cronbach's alpha on quiz stats

fixes CNVS-11518
On quizzes with larger groups of participants, the alpha
generated by QuizStatistics has been reported as being
off by quite a bit.  This was because of a faulty
float value that was used to determine the 'middle' range.

Test plan
- Not easily testable by QA.

Change-Id: I0a9d925b2770eb224de6bbbf43bd2453177e4377
Reviewed-on: https://gerrit.instructure.com/31124
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Josh Simpson <jsimpson@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>