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: NRené Scharfe <l.s.r@web.de>
Reviewed-by: NHeiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: NNick Townsend <nick.townsend@mac.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/technical/api-ref-iteration.txt

@@ -50,10 +50,10 @@ submodules object database. You can do this by a code-snippet like
 this:
 
 	const char *path = "path/to/submodule"
-	if (!add_submodule_odb(path))
+	if (add_submodule_odb(path))
 		die("Error submodule '%s' not populated.", path);
 
-`add_submodule_odb()` will return an non-zero value on success. If you
+`add_submodule_odb()` will return zero on success. If you
 do not do this you will get an error for each ref that it does not point
 to a valid object.