doc: add missing public access modifier.

Signed-off-by: Squareys <Squareys@googlemail.com>

src/Magnum/SceneGraph/Drawable.h

@@ -50,7 +50,7 @@ draws blue sphere:
 typedef SceneGraph::Object<SceneGraph::MatrixTransformation3D> Object3D;
 typedef SceneGraph::Scene<SceneGraph::MatrixTransformation3D> Scene3D;
 
-class RedCube: public Object3D, SceneGraph::Drawable3D {
+class RedCube: public Object3D, public SceneGraph::Drawable3D {
     public:
         explicit RedCube(Object3D* parent, SceneGraph::DrawableGroup3D* group): Object3D{parent}, SceneGraph::Drawable3D{*this, group} {
             std::tie(_mesh, _vertices, _indices) = MeshTools::compile(Primitives::UVSphere::solid(16, 32));