replaced another dead defelement link with valid one

[jfdev001]

Related to issue #1142,

grep -Rnow . -e "defelement"

shows that the only remaining place where a dead defelement link may exist is in ./docs/src/topics/FEValues.md:179:defelement . I replace this link with the valid link at the current website, and I do not use a wayback link here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.59%. Comparing base (7ff967c) to head (f5c0afd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1148   +/-   ##
=======================================
  Coverage   93.59%   93.59%           
=======================================
  Files          39       39           
  Lines        6087     6087           
=======================================
  Hits         5697     5697           
  Misses        390      390           

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