Add `prezto submodules update` to zsh/update so prezto gets cloned with all its submodules.
Add `prezto submodules update` to zsh/update so prezto gets cloned with all its submodules.

thezimmee/os-zimmee,thezimmee/os-zimmee,thezimmee/os-zimmee,thezimmee/os-zimmee,thezimmee/os-zimmee,thezimmee/os-zimmee