Fix shader_instancing example (#9448)

# Objective

- Fix shader compilation issue in `shader_instancing` example.

## Solution

- Fix typo in `instancing.wgsl`

assets/shaders/instancing.wgsl

@@ -24,7 +24,7 @@ fn vertex(vertex: Vertex) -> VertexOutput {
  // index in the Mesh array. This index could be passed in via another
  // uniform instead but it's unnecessary for the example.
  out.clip_position = mesh_position_local_to_clip(
- get_model_matrix(0),
+ get_model_matrix(0u),
  vec4<f32>(position, 1.0)
  );
  out.color = vertex.i_color;