Update computed properties example

[NataliaTepluhina]

Changed an example of computed properties to be less cryptic and more 'real-life'

Close #38

[bencodezen]

@NataliaTepluhina This is great! Love the result.

Just had a few suggestions for consistency amongst the book titles, but otherwise you're good to go!

[CyberAP]

Would it be reasonable to add an Add book functionality to highlight that computed properties are in fact reactive? That could potentially help to understand why book list was put in data rather than in a variable outside app.