fixing line comment in 2d_shapes.rs example (#12865)

# Objective

- There is a little mistake in a line comment.

## Solution

- Fixed the comment to correctly describe what happens in the documented
calculation.

examples/2d/2d_shapes.rs

@@ -44,7 +44,7 @@ fn setup(
             mesh: shape,
             material: materials.add(color),
             transform: Transform::from_xyz(
-                // Distribute shapes from -X_EXTENT to +X_EXTENT.
+                // Distribute shapes from -X_EXTENT/2 to +X_EXTENT/2.
                 -X_EXTENT / 2. + i as f32 / (num_shapes - 1) as f32 * X_EXTENT,
                 0.0,
                 0.0,