It looks like materialdesignicons.com removed the icon formerly at https://materialdesignicons.com/icon/currency-usd-circle-outline
The un-circled dollar sign at https://materialdesignicons.com/icon/currency-usd might be a viable alternative (or, obviously, code/chapter-3/app/src/main/res/drawable/mdi_coin_black_24dp.xml from the archive zip)