plotting example mimo system about python-control

[murrayrm]

A couple of issues:

• The documentation you are looking at is for the latest, which is the main development branch. That version of python-control has significant changes to the various plotting routes (see #917 and the PR's linked there). For version 0.9.4, you want to use https://python-control.readthedocs.io/en/0.9.4 (the specific section you address is not there).
• For version 0.9.4 of python-control, the frequency_response function requires omega; see 0.9.4 documentation.

Ways to fix the problem:

• If you want to stick with version 0.9.4, you can generate a bode plot using bode_plot([sys1, sys2]).
• If you want to use the update _response/_plot pattern, checkout the main branch on GitHub.