Add examples of fungible items for footnote 2

[jorgesumle]

They were missing. I took "tin" from https://wiki.mises.org/wiki/Fungible, which is what this book references, and "gold" from https://en.wikipedia.org/wiki/Fungibility. I don't think grain and coal are the best examples, because grain can go bad and there are different types of coal, that's why I didn't choose those.

[jorgesumle]

I would also update this:

Monero is an established cryptocurrency with over five years of development

Now it's more than nine years.

[jorgesumle]

Same here:

A previous attempt to explain how Monero works [110] did not elucidate elliptic curve cryptography, was
incomplete, and is now over five years outdated

Over nine years now.