change link text to be more descriptive

change link text from "Switch gradebooks"
to "Switch to Default Gradebook"

fixes CNVS-15550

test plan:
- Navigate to the gradebook and click on the
  link in the upper right-hand corner titled
  "Switch to Individual View"
- Once switched to the individual view, verify
  that there is a link at the top of the page
  titled "Switch to Default Gradebook"
- Click on the "Switch to Default Gradebook"
  link and verify that it navigates you back
  to the default gradebook view.

Change-Id: Icbe0b12e674a3f4338a025e20bb6909d6b419502
Reviewed-on: https://gerrit.instructure.com/42277
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>