Mistake in help string for `last name` argument

Help string for `last name` argument duplicate help string for `first name` argument [there](https://github.com/PacktPublishing/Rust-Web-Programming-2nd-Edition/blob/main/chapter02/interacting_with_cargo/web_app_with_clap/src/main.rs#L18):

```rs
        .help("first name of user")
```