Update how output of `matplotlib` is shown

[Cheukting]

As Pyscirpt is in active development, the way how matplotlib result is handled changes. Need to:

• use display() to show fig in Chapter2 exercise 1 and;
• display(fig, target="output") for exercise 3

(see https://docs.pyscript.net/latest/reference/API/display.html?highlight=display)

Since display does not know how to replay already existing child properly (maybe worth reporting this issue to pyscript), we need to do a Element("output").clear() before display