cloud-init: fix spelling mistake in error message

Observed this becuase cloud-init fails on my setup.

Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>