docs: Fix demo help

[adamchainz]

It gets pulled from the comment here: https://github.com/clap-rs/clap/blob/master/examples/demo.rs#L3

[epage]

The about attribute is overriding the doc comment.

Because of the role of demo we want to show about but we likely don't want it to show clap's package.documentation.

One option is to remove the doc comment and add the following after the existing about

# #[command(about = "Simple program to greet a person")] // emulating implicit about`