Doc: corrects obsolete pointer syntax

[gamazeps]

Goes from ~ to box
Closes #18551

[steveklabnik]

Thank you!!!

[gereeter]

Shouldn't it be vec![], not box []?

[gamazeps]

@gereeter Quite honestly, I just updated the content of the doc to the new syntax, but yes, it seems that the old doc was a bit weird.
If you're sure it's should be vec![], I'll gladly change it (as I'm not really confident in what I changed (let's say that I'm sure that at least it's not wronger than it used to be))

[brendanzab]

I can confirm that it should be vec![]. The variant is defined as: PatVec(Vec<P<Pat>>, Option<P<Pat>>, Vec<P<Pat>>),

[steveklabnik]

Yes, technically Box<[T, ..N]> is a type, but is less useful than Vec<T>.