Decrease SphereShape3D's default radius to 0.5 to match primitive mesh

The SphereMesh primitive mesh's size was recently decreased, but
unlike other primitive meshes, the sphere shape's radius wasn't
adjusted accordingly.

doc/classes/SphereShape3D.xml

@@ -11,7 +11,7 @@
 		<link title="3D Physics Tests Demo">https://godotengine.org/asset-library/asset/675</link>
 	</tutorials>
 	<members>
-		<member name="radius" type="float" setter="set_radius" getter="get_radius" default="1.0">
+		<member name="radius" type="float" setter="set_radius" getter="get_radius" default="0.5">
 			The sphere's radius. The shape's diameter is double the radius.
 		</member>
 	</members>

scene/resources/sphere_shape_3d.cpp

@@ -83,5 +83,5 @@ void SphereShape3D::_bind_methods() {
 
 SphereShape3D::SphereShape3D() :
 		Shape3D(PhysicsServer3D::get_singleton()->shape_create(PhysicsServer3D::SHAPE_SPHERE)) {
-	set_radius(1.0);
+	set_radius(0.5);
 }

scene/resources/sphere_shape_3d.h

@@ -35,7 +35,7 @@
 
 class SphereShape3D : public Shape3D {
 	GDCLASS(SphereShape3D, Shape3D);
-	float radius = 1.0f;
+	float radius = 0.5f;
 
 protected:
 	static void _bind_methods();