Improve flip example in docs

[milesfrain]

The existing example for flip with const can be pretty confusing for beginners. See purescript-contrib/purescript-book#260

Let's use a clearer example, like div or sub.

[natefaubion]

There is probably a better example than const, but the reason I think it's used, and something that I think is really important, is that it's easy to see by the types what is happening.

const :: forall a b. a -> b -> a
flip const :: forall a b. b -> a -> a

Whereas, something like sub might be

sub :: Int -> Int -> Int
flip sub :: Int -> Int -> Int

[milesfrain]

Then the existing example with const should probably be modified to use different types to showcase this feature.

I also think we should include a sub/div example too.

Any objections to multiple examples?

[hdgarrood]

I think multiple examples is fine as long as it doesn’t make the docs too long. Adding a sentence to say something like “flip is useful when you want to partially apply arguments in a different order to the order expected by the function you’re using” would probably be good too.