rtchallenge: Address cargo clippy.

rtchallenge/src/canvas.rs

@@ -48,7 +48,7 @@ impl Canvas {
     {
         let path = Path::new(path.as_ref());
         let file = File::create(path)?;
-        let ref mut w = BufWriter::new(file);
+        let w = &mut BufWriter::new(file);
 
         let mut encoder = png::Encoder::new(w, self.width as u32, self.height as u32);
         encoder.set_color(png::ColorType::RGB);