Hi, I know it’s a bit late to answer this, but I hope it might help someone else who faces the same situation.
I had the same problem, and the solution was to update the docs library.
I guess the ones in the book aren’t compatible with newer Elixir versions.
I ended up using:
{:ex_doc, "~> 0.29.1"},
{:earmark, "~> 1.4.33"},