Careful, you have to also add --no-preserve-root to make sure you get all of it out. If you leave the roots, it'll just grow back later!
(But seriously, don't actually do this unless you're prepared to lose data and potentially even brick your computer. Don't even try it on a VM or a computer you're planning to wipe anyway, because if something is mounted that you don't expect, you'll wipe that too. On older Linux kernels, EFI variables were mounted as writable, so running rm -rf /could actually brick your computer. This shouldn't still be the case, but I wouldn't test it, myself.)