Fixed broken link for HTM

[hhy2022]

Same as #1031 but on the FESTIM 2 docs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.95%. Comparing base (2830e18) to head (c6c93da).
⚠️ Report is 80 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx    #1033   +/-   ##
========================================
  Coverage    94.95%   94.95%           
========================================
  Files           44       44           
  Lines         3111     3111           
========================================
  Hits          2954     2954           
  Misses         157      157           

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