fix transfer function colon indexing

[baggepinnen]

Closes #871

[codecov[bot]]

Codecov Report

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/JuliaControl/ControlSystems.jl/pull/872/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaControl)

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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
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