Correct legend in README plot

[baggepinnen]

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[JuliaControlBot]

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Plots were compared to references. 8/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

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[codecov]

Codecov Report

Merging #366 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #366   +/-   ##
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  Coverage   82.42%   82.42%           
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  Files          31       31           
  Lines        2822     2822           
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  Hits         2326     2326           
  Misses        496      496           

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[mfalt]

We should probably just remove the TransferFunction[...] notation also. It was needed in some early version of the package to avoid creating a mimo system