Fix inaccurate comment in custom_ui_material.wgsl shader (#16846)

# Objective

- Modify a comment in the shader file to describe what the shader
actually does
- Fixes #16830

## Solution

- Changed the comment.

## Testing

- Testing is not relevant to fixing comments (as long as the comment is
accurate)

---------

Co-authored-by: Freya Pines <freya@MacBookAir.lan>
Co-authored-by: Freya Pines <freya@Freyas-MacBook-Air.local>

assets/shaders/custom_ui_material.wgsl

@@ -1,4 +1,4 @@
-// This shader draws a circle with a given input color
+// Draws a progress bar with properties defined in CustomUiMaterial
 #import bevy_ui::ui_vertex_output::UiVertexOutput
 
 @group(1) @binding(0) var<uniform> color: vec4<f32>;