ref-iteration doc: add_submodule_odb() returns 0 for success

The usage sample of add_submodule_odb() function in the Submodules
section expects non-zero return value for success, but the function
actually reports success with zero.

Helped-by: René Scharfe <l.s.r@web.de>
Reviewed-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Nick Townsend <nick.townsend@mac.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git 1.8.5.1

Signed-off-by: Junio C Hamano <gitster@pobox.com>