[libc++][docs] Mark LWG3447 as complete

Mark LWG3447 as complete since it was not an issue since the original
implementation of `take_view` from
0f4b41e0. Currently, `take_view`'s
deduction guide does not constrain the range on the `range` concept.

Reviewed By: ldionne, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D111501