I should've done this in #26 but didn't.

Besides fixing that error, this commit improves the output in the
notebooks in two other ways:

1. The Graphviz version used in this run handled some things
   better. See #5 and #6. (This is due to the system Graphviz on
   the machine I'm using to rerun this being old enough to have
   the bug discussed there. Using the conda environment instead of
   the poetry environment addresses that, because in the conda
   environment, the graphviz conda package is used.)

2. One of the Git repositories being visualized was absent from
   the test machine before. I cloned it so that graph would be
   redrawn rather than omittedw with an error.