Fix font-extension URLs to include @mathjax (mathjax/MathJax#3387)

[dpvc]

This PR fixes an error with the font-extension URLs, which failed to include the @mathjax scope.

Resolves issue mathjax/MathJax#3387.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (4d1e195) to head (33244ad).
Report is 14 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1291   +/-   ##
========================================
  Coverage    86.71%   86.71%           
========================================
  Files          337      337           
  Lines        83979    83979           
  Branches      4750     4750           
========================================
  Hits         72826    72826           
  Misses       11130    11130           
  Partials        23       23           

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[zorkow]

lgtm.