This fix is based on the comment in
https://github.com/docker/docker/pull/27567#discussion_r86910604
Basically, in the help output of `docker service update`, the `--xxx-add`
flags typically have plural forms while `--xxx-rm` flags have singular
forms.
This fix updates the help output for consistency.
This fix also updates the related docs in `service_update.md`.
The help output in `service_update.md` has been quite out-of-sync
with the actual output so this fix replaces the output with the
most up-to-date output.
This fix is related to #27567.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>