Add missing `view_formats`

In "Depth Buffer" chapter.

docs/beginner/tutorial8-depth/README.md

@@ -41,6 +41,7 @@ impl Texture {
             format: Self::DEPTH_FORMAT,
             usage: wgpu::TextureUsages::RENDER_ATTACHMENT // 3.
                 | wgpu::TextureUsages::TEXTURE_BINDING,
+            view_formats: &[],
         };
         let texture = device.create_texture(&desc);