Parser::custom_usage combine and derive examples are swapped

[TBBle]

Noted while reading the docs, https://github.com/pacak/bpaf/blob/master/docs2/src/custom_usage/combine.rs is a derive example, and https://github.com/pacak/bpaf/blob/master/docs2/src/custom_usage/derive.rs is a combine example.

[pacak]

Oops. This what happens when you have over a hundred of them. I appreciate the bug report ❤️ will look into fixing it and will check if there's more such cases.