[libc++][ranges][NFC] Add contiguous_iterator_tag to the <iterator> synopsis

It was previously implemented by commit [45d048c2](https://reviews.llvm.org/rG45d048c20440989df2b4e1be1f9343225e7741ab).

Reviewed By: #libc, Quuxplusone, philnik, ldionne

Differential Revision: https://reviews.llvm.org/D118751