Rewrite `Effects`'s paragraph with impure function argument

[ninaandrup]

The page Effects has this paragraph which explains "A higher-order function can also require that a function is impure.". But the syntax ~> used in this example

def foreach(f: a ~> Unit, l: List[a]): Unit & Impure

is not allowed anymore. And the paragraph should be rewritten.

[ninaandrup]

This table also specifies that the shorthand notation a ~> b can be used instead of a -> b & Impure.

[magnus-madsen]

This can no longer be required and so the text should be removed :)