i18n fixes for content_exports/index.html

fixes gh-476

Change-Id: I73481c0c0571184e2656bd0d9a23bfdaf84c330c
Reviewed-on: https://gerrit.instructure.com/37858
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>