Use a non-deprecated icon name for notes

This fixes warnings when building PDFs:
INFO: note admonition in theme uses icon from deprecated fa icon set; use fas, far, or fab instead

Looking at prawn-icon, far-edit is the correct replacement:
https://github.com/jessedoyle/prawn-icon/blob/e76a2934237f97121fe93223cf5c71499624a594/data/fonts/fa4/shims.yml#L294C1-L295C1

Fixes https://github.com/riscv/docs-resources/issues/6