update example to work on repl

[intronic]

Hi .. thanks for this and the example, it was very helpful to a newcomer.
Please consider these additions which may be helpful to others in my situation.

I had some head scratching to find some things like:

• where these symbols were: /, ? (I found a suggestion in the issues here about adding it to the docs, so thought I would)

• how to get cru function to compile (found a thread on discourse about adding a constraint being one easy way to resolve it)

• where _Newtype was

• I also added the generic show inline as if you're new to this you need to keep referring to other docs to remember details

Cheers,

Mike