Improve instancing layout comments order

I find it clearer if the comment explaining why 4 slots are needed is moved before the first slot definition.

docs/beginner/tutorial7-instancing/README.md

@@ -148,6 +148,8 @@ impl InstanceRaw {
             // instance when the shader starts processing a new instance
             step_mode: wgpu::VertexStepMode::Instance,
             attributes: &[
+                // A mat4 takes up 4 vertex slots as it is technically 4 vec4s. We need to define a slot
+                // for each vec4. We'll have to reassemble the mat4 in the shader.
                 wgpu::VertexAttribute {
                     offset: 0,
                     // While our vertex shader only uses locations 0, and 1 now, in later tutorials we'll
@@ -155,9 +157,6 @@ impl InstanceRaw {
                     shader_location: 5,
                     format: wgpu::VertexFormat::Float32x4,
                 },
-                // A mat4 takes up 4 vertex slots as it is technically 4 vec4s. We need to define a slot
-                // for each vec4. We'll have to reassemble the mat4 in
-                // the shader.
                 wgpu::VertexAttribute {
                     offset: mem::size_of::<[f32; 4]>() as wgpu::BufferAddress,
                     shader_location: 6,