Fix python error in introduction.md

[garyrob]

It had:

let compose(f: Callable[[A], B], g: Callable[[B], C]) -> Callable[[A], C]:
    lambda x: g(f(x))

And should have been:

def compose(f: Callable[[A], B], g: Callable[[B], C]) -> Callable[[A], C]:
    return lambda x: g(f(x))

[dbrattli]

Thanks for fixing!