Const generics - Known issues doc comment

[yancyribbens]

There's a doc comment that appears outdated:

Therefore, until const generics lands in stable or when
// we can remove the `T: Debug` bound on Arbitrary, we can not support arrays
// sized over 32.

const generics landed in 1.51 from what I can tell (well past the current proptest MSRV) https://blog.rust-lang.org/2021/02/26/const-generics-mvp-beta.html. Anyway, maybe this comment can be removed or updated.

comment:
https://github.com/proptest-rs/proptest/blob/master/proptest-derive/src/lib.rs#L26