Change `Maybe.andThen` docs to use `List.head` over `String.toInt`

[rtfeldman]

The current docs for Maybe.andThen use the example of String.toInt, but that function now returns a Result rather than a Maybe.

This revises the docs to use List.head for the example instead, as it does return a Maybe:

{-| Chain together many computations that may fail. It is helpful to see its
definition:

    andThen : Maybe a -> (a -> Maybe b) -> Maybe b
    andThen maybe callback =
        case maybe of
            Just value ->
                callback value

            Nothing ->
                Nothing

This means we only continue with the callback if things are going well. For
example, say you need to use (`head : List Int -> Maybe Int`) to get the
first month from a `List` and then make sure it is between 1 and 12:

    toValidMonth : Int -> Maybe Int
    toValidMonth month =
        if month >= 1 && month <= 12 then
            Just month
        else
            Nothing

    getFirstMonth : List Int -> Maybe Int
    getFirstMonth months =
        head months `andThen` toValidMonth

If `head` fails and results in `Nothing` (because the `List` was empty`),
this entire chain of operations will short-circuit and result in `Nothing`.
If `toValidMonth` results in `Nothing`, again the chain of computations
will result in `Nothing`.

This also updates the code examples to use the current style recommendations.

[evancz]

Thanks! :D

[evancz]

Thanks! :D