Improve the inheritance example to something more meaningful (was remove TODO)

[AlexDaniel]

The docs have a TODO notice written right in the text: http://doc.perl6.org/language/classtut#Inheritance

See also: Raku/book#58

[jonathanstowe]

Frankly I would just take the redundant TODO out and change the title of this issue

[AlexDaniel]

@jonathanstowe go for it.

[jonathanstowe]

In summary the discussion that gave rise to the original issue was that the example provided was a little contrived and lacked a clear rationale.

A good example should define a problem and show how it can be dealt with using inheritance.

[JJ]

I would say this is no longer relevant. Maybe old issues should provide a link to particular commits in the repo.
In this case, TODO has been removed. Close?

[JJ]

I see a couple of not very good practices (using say in methods) here and also a way to include in the example the fact that attributes are always private. I can change that if needed...