bugfix ubuntu's release name

[cboettig]

20.04 is focal, bionic is is 18.04 :-).

Absolutely fantastic book here. Have never found a treatment of this material do such a good job of including the right details while also not getting buried in arcana.

[akgold]

Thank you for the compliments and the fix! Insert comment here about feeling old. 😆

If you wouldn't mind, could you take a look at https://github.com/akgold/do4ds/blob/main/CONTRIBUTING.md and respond with a comment with the phrase there. Thanks again!

[cboettig]

I assign the copyright of this contribution to Alex Gold.

great work here, look forward to sharing this book with my students.

[akgold]

Fantastic. Thanks @cboettig!