DOCS: Fix is_dielectric docstring

[Samuelopez-ansys]

Description

Fix wrong definition.

Issue linked

Close #6671

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have agreed with the Contributor License Agreement (CLA).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.45%. Comparing base (0a3d352) to head (5765b36).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6677      +/-   ##
==========================================
+ Coverage   82.34%   82.45%   +0.10%     
==========================================
  Files         242      242              
  Lines       76995    77004       +9     
==========================================
+ Hits        63401    63490      +89     
+ Misses      13594    13514      -80     

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