use correct canvadocs attribute in speedgrader

By sheer luck, canvadocs still works, but this is the way it was
supposed to be written.

Test plan:
  Make sure canvadocs previews work in speedgrader

Change-Id: I849b7d2d29a3a6f3d6973331f2745684d1dab7e1
Reviewed-on: https://gerrit.instructure.com/36193
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>