Fix CI error on new color grading example (#13180)

# Objective

Fixes https://github.com/bevyengine/bevy/issues/13179

## Solution

Obey clippy.

## Commentary

I'm really confused why CI didn't fail on the initial PR merge here.

Co-authored-by: Alice Cecile <alice.i.cecil@gmail.com>

examples/3d/color_grading.rs

@@ -639,7 +639,7 @@ fn update_ui_state(
         {
             if let Some(ref value_label) = value_label {
                 for section in &mut text.sections {
-                    section.value = value_label.clone();
+                    section.value.clone_from(value_label);
                 }
             }
         }