fix transfer function colon indexing

[baggepinnen]

Closes #871

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +1.51% 🎉

Comparison is base (6540a25) 91.21% compared to head (d8a077a) 92.72%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
+ Coverage   91.21%   92.72%   +1.51%     
==========================================
  Files           5        5              
  Lines         330      330              
==========================================
+ Hits          301      306       +5     
+ Misses         29       24       -5     

see 1 file with indirect coverage changes

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[JuliaControlBot]

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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.

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