Documentation: seccomp: unify list indentation

Use tabs to indent SECCOMP_RET_USER_NOTIF definition, for consistency
with other items in this list.

Signed-off-by: Jakub Wilk <jwilk@jwilk.net>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>