message: Fix doc example in `pure::canvas::Program`
  The `widget` module was missing.
diff: @@ -28,8 +28,10 @@ use std::marker::PhantomData;
  /// ```no_run
  /// # mod iced {
  /// # pub mod pure {
  +/// # pub mod widget {
  /// # pub use iced_graphics::pure::canvas;
  /// # }
  +/// # }
  /// # pub use iced_native::{Color, Rectangle, Theme};
  /// # }
  /// use iced::pure::widget::canvas::{self, Canvas, Cursor, Fill, Frame, Geometry, Path, Program};
  