message: use explicit cursor import
diff: //! Load images using [`io::Reader`]:
  //!
  //! ```rust,no_run
  -//! # use std::io::Cursor;
  //! use image::io::Reader as ImageReader;
  //! # fn main() -> Result<(), image::ImageError> {
  //! # let bytes = vec![0u8];
  //!
  //! let img = ImageReader::open("myimage.png")?.decode()?;
  -//! let img2 = ImageReader::new(Cursor::new(bytes)).decode()?;
  +//! let img2 = ImageReader::new(std::io::Cursor::new(bytes)).decode()?;
  //! # Ok(())
  //! # }
  //! ```
  