fix some linting warnings

[baggepinnen]

No description provided.

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 11/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
❌ 0.048
✔️ 0.001
✔️ 0.001
✔️ 0.001
✔️ 0.001
✔️ 0.0
✔️ 0.0
❌ 0.034
❌ 0.06
✔️ 0.0
✔️ 0.01

[codecov]

Codecov Report

Merging #753 (715b8e9) into master (ce16403) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #753   +/-   ##
=======================================
  Coverage   94.48%   94.48%           
=======================================
  Files           3        3           
  Lines         290      290           
=======================================
  Hits          274      274           
  Misses         16       16           

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