In #24159, the title field of `docker node ls` has been
changed from NAME to HOSTNAME. However, in the docs the
NAMEs are still used for the output of `docker node ls`.
This fix updates docs so that NAME field is changed to
HOSTNAME for all `docker node ls`.
This fix is related to #24159 and #24090.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>