Error in Scope documentation example

Whilst I don't completely grasp Scopes, I believe the example given in the docs has a small mistake that makes it fundamentally incorrect.

The example as it stands:
```rust
use druid::{Data, Lens, WidgetExt};
use druid::widget::{TextBox, Scope};
#[derive(Clone, Data, Lens)]
struct AppState {
name: String,
}

#[derive(Clone, Data, Lens)]
struct PrivateState {
text: String,
other: u32,
}

impl PrivateState {
pub fn new(text: String) -> Self {
PrivateState { text, other: 0 }
}
}

fn main() {
let scope = Scope::from_lens(
PrivateState::new,
PrivateState::text,
TextBox::new().lens(PrivateState::text),
);
}
```

I believe the fourth to last line uses the wrong lens - the example is attempting to demonstrate the use of a lens on _the application state_ as a means of transferring start state into the private state. As it is, it just looks like circular references of private states initialised by other private states.

I believe the main function should instead look like this:
```rust
fn main() {
let scope = Scope::from_lens(
PrivateState::new,
AppState::name,
TextBox::new().lens(PrivateState::text),
);
}
```

Review the Scope documentation example and verify how its constructor and lens arguments relate the application and private state. Check the example with the relevant Rust documentation or build process; it is done when the corrected example uses the application-state lens and remains valid.

Written by the indexing model from the issue text.