Modifies the footer alt text to match the image

fixes CNVS-14590

Test Plan:
  - Using a screenreader, go to any page in Canvas,
    even the login page.
  - When you navigate to the footer "By Instructure" the
    screenreader should simply read "By Instructure"

Change-Id: Ie61711610ab93f17792c0d288c1069fc0283a4e6
Reviewed-on: https://gerrit.instructure.com/40456
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>