Gdnative cpp example: variable visibility

I think I've found a little error in [gdnative-cpp-example.rst](https://github.com/godotengine/godot-docs/blob/master/tutorials/plugins/gdnative/gdnative-cpp-example.rst#adding-properties). When showing how to create a new property without getter and setter, its visibility is set to private
`
...
private:
float time_passed;
float amplitude;
...
`
but this prevent the Godot interface from accessing the variable itself (which then is not visible in the inspector).