Covariance example could be clearer

[ChrisBrooksbank]

'''c#
circlePusher.Push( new Circle());
Shape aShape = shapePopper.Pop();
'''

Implys we push a Circle onto a stack, and then pop it back off.
In fact this isn't the case as its two separate stack objects.

Also we ought to mention the naiveness of the Stack class, i.e. no bounds checking