Fixed missing `pub` in ModelVertex struct

docs/beginner/tutorial9-models/README.md

@@ -13,9 +13,9 @@ pub trait Vertex {
 #[repr(C)]
 #[derive(Copy, Clone, Debug, bytemuck::Pod, bytemuck::Zeroable)]
 pub struct ModelVertex {
-    position: [f32; 3],
-    tex_coords: [f32; 2],
-    normal: [f32; 3],
+    pub position: [f32; 3],
+    pub tex_coords: [f32; 2],
+    pub normal: [f32; 3],
 }
 
 impl Vertex for ModelVertex {