Utilise new method for 2d circle example. (#15022)

# Objective
Replaced the existing instantiation of the 2D Circle in the 2d_shapes.rs
file with the `new` method.
- Ensures consistency in instantiating 2D primitive shapes in the
examples.

## Solution
Replaced the existing instantiation of the 2D Circle in the 2d_shapes.rs
file with the `new` method.
- Ensures consistency in instantiating 2D primitive shapes in the
examples.

## Testing
- None: Should be straight-forward enough to not warrant a test (I will
eat my words if I am wrong).

---

examples/2d/2d_shapes.rs

@@ -33,7 +33,7 @@ fn setup(
     commands.spawn(Camera2dBundle::default());
 
     let shapes = [
-        Mesh2dHandle(meshes.add(Circle { radius: 50.0 })),
+        Mesh2dHandle(meshes.add(Circle::new(50.0))),
         Mesh2dHandle(meshes.add(CircularSector::new(50.0, 1.0))),
         Mesh2dHandle(meshes.add(CircularSegment::new(50.0, 1.25))),
         Mesh2dHandle(meshes.add(Ellipse::new(25.0, 50.0))),