docs: `split-debuginfo` docs claim `unpacked` is the default on macOS.

See https://github.com/rust-lang/cargo/pull/9112#discussion_r584767537 - I believe the rest of the PR was updated to keep the default `packed` (see https://github.com/rust-lang/cargo/pull/9112/commits/ffa9dbd34853457fd07f5c7152dc03adee069c96), but docs were missed.

This is now in beta so the doc fix would have to be backported to that as well.

cc @ehuss @alexcrichton 