[BUG] Update the wording in the qubit rotation demo

[CatalinaAlbornoz]

The optimization section of the qubit rotation demo is worded as if the demo was still using the default NumPy/Autograd interface. We need to update the wording to actually match the demo, which now uses JAX. There may be other inconsistencies in other parts of the demo, it would be good to give it a full read and update.