[markov_chains_II] Example Admonition in Markov Chain II is not properly defined

[HumphreyYang]

(Please leave this open as I would like to fix this on live on Friday's workshop for XMU RAs)

The example admonition is not well defined in markov_chains_II:

We should switch it to

````{prf:example}
````

[HumphreyYang]

Hi @mmcky and team,

Please leave this open as I want to use this as a live demo example on Friday.

It shows how to update lectures and teaches some MyST syntax concepts (i.e. nested admonitions)!