Fix negative phase margin

[baggepinnen]

Closes #961

[JuliaControlBot]

Something failed when generating plots. See the log at https://github.com/JuliaControl/ControlExamplePlots.jl/actions/runs/12962779755?check_suite_focus=true for more details.

[JuliaControlBot]

This is an automated message.
Plots were compared to references. No changes were detected.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.78%. Comparing base (987c0ec) to head (bdbeb80).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #962   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
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  Files          41       41           
  Lines        4975     4975           
=======================================
  Hits         4616     4616           
  Misses        359      359           

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