Update index.md

[baggepinnen]

[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
:x: 0.031
:warning: 0.025
:heavy_check_mark: 0.008
:warning: 0.019
:warning: 0.024
:warning: 0.025
:warning: 0.017
:warning: 0.023
:x: 0.032
:heavy_check_mark: 0.006
:heavy_check_mark: 0.01

[codecov[bot]]

Codecov Report

Merging #466 (db28b6a) into master (484b00f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #466   +/-   ##
=======================================
  Coverage   85.20%   85.20%           
=======================================
  Files          31       31           
  Lines        3102     3102           
=======================================
  Hits         2643     2643           
  Misses        459      459           

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