docs: Update README section on `exa` to include `eza`

[mikesmithgh]

exa is unmaintained now and eza is the successor

https://github.com/ogham/exa https://github.com/eza-community/eza

The README docs provide a comparison between exa and erd. https://github.com/solidiquis/erdtree#exa

You should probably update the README to reference eza now, or both.