Consider changing stringsAsFactors and 'master' branch references in course materials

[amoeba]

We have various references to stringsAsFactors which we might want to take out at some point. Maybe not yet as R 4.0 is still fairly new?
We also have references to the master branch in at least the [web publication chapter])https://learning.nceas.ucsb.edu/2021-02-RRCourse/session-9-data-visualisation-and-publishing-to-the-web.html).

Might be good to grep the modules and change some of these.