bug#34010: "guix pull -h" does not show --system option

[Alex Kost]

Hello, I have noticed that commit
5923102f7b58f0a0120926ec5b81ed48b26a188e did not add a notice of
"--system" option to 'show-help' procedure, so "guix pull -h" does not
show this option.

-- 
Alex