docs: Update sandbox link in studio footer

[sarina]

I specified the wrong URL for the DemoX course on openedx/platform-roadmap#440 (comment).

@brian-smith-tcril do I need to bump a version here so we can get this change into the Authoring MFE?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.12%. Comparing base (39a8824) to head (5722746).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #561   +/-   ##
=======================================
  Coverage   95.12%   95.12%           
=======================================
  Files          14       14           
  Lines          82       82           
  Branches       18       18           
=======================================
  Hits           78       78           
  Misses          4        4           

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[sarina]

Closing to kick the tests

[brian-smith-tcril]

I need to bump a version here so we can get this change into the Authoring MFE?

I can land it as a fix: on squash so it triggers a release.