message: fix progress bar docs to no longer mention buttons
diff: -//! Allow your users to perform actions by pressing a button.
  +//! Allow your users to visually track the progress of a computation.
  //!
  -//! A [`Button`] has some local [`State`].
  +//! A [`ProgressBar`] has a range of possible values and a current value,
  +//! as well as a length, height and style.
  //!
  -//! [`Button`]: type.Button.html
  -//! [`State`]: struct.State.html
  +//! [`ProgressBar`]: type.ProgressBar.html
  use crate::Renderer;
  pub use iced_style::progress_bar::{Style, StyleSheet};
  