Update chapter11.md

[theRohan2001]

I think we have to explicitly call the shape's update function in both the shapes and also in the main.lua because it wasn't working for me and I thought you don't have to call the functions. We should add that to avoid further confusion. Thank you

[Sheepolution]

You don't need to have Rectangle:update and Circle:update. If a class doesn't have a method it calls its parent's method instead.