Need to mention the 'graphviz' library

[beckykd]

URL to the relevant documentation

https://docs.quantum.ibm.com/transpile/custom-backend#visualize-backends

Select all that apply

• [ ] typo
• [ ] code bug
• [ ] out-of-date content
• [ ] broken link
• [X] other

Describe the fix.

Plotting a coupling map requires the graphviz library. We should list it as a requirement in this topic.

On Mac it seems simple to install, but on Windows, there's a whole big process. I didn't have time to investigate further, but wanted to note the requirement.

MissingOptionalLibraryError: "The 'graphviz' library is required to use 'plot_coupling_map'. You can install it with ''brew install graphviz' if on Mac, or by downloding it from their website'."

[javabster]

related to #1286