Fix ifProgress example

[MethodGrab]

The ifProgress example does not compile:

The 1st argument to `andThen` is not what I expect:

33|     |> andThen (\newOffset -> if offset == newOffset then Done () else Loop newOffset)
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This argument is an anonymous function of type:

    Int -> Step Int ()

But `andThen` needs the 1st argument to be:

    Int -> Parser b

andThen should be map.