Add: Further example to List.indexedMap to clarify

[benkoshy]

What is the change?

It adds another example that will hopefully be clearer to users.

Why the change?

For me, the existing example was not altogether clear. (I am relatively new Elm user.) I did some Googling and found out that other users were facing the exact same issue: https://github.com/elm/core/issues/1008

The Solution: Add another example