fix(typos): tutorial5 change documentation

can't perform a unwrap on to_rgba8

docs/beginner/tutorial5-textures/README.md

@@ -35,7 +35,7 @@ surface.configure(&device, &config);
 
 let diffuse_bytes = include_bytes!("happy-tree.png");
 let diffuse_image = image::load_from_memory(diffuse_bytes).unwrap();
-let diffuse_rgba = diffuse_image.to_rgba8().unwrap();
+let diffuse_rgba = diffuse_image.as_rgba8().unwrap();
 
 use image::GenericImageView;
 let dimensions = diffuse_image.dimensions();