Add zsh completion for 'dockerd --shutdown-timeout'

Signed-off-by: Steve Durrheimer <s.durrheimer@gmail.com>

contrib/completion/zsh/_docker

@@ -1625,6 +1625,7 @@ __docker_subcommand() {
                 "($help)*--registry-mirror=[Preferred Docker registry mirror]:registry mirror: " \
                 "($help -s --storage-driver)"{-s=,--storage-driver=}"[Storage driver to use]:driver:(aufs btrfs devicemapper overlay overlay2 vfs zfs)" \
                 "($help)--selinux-enabled[Enable selinux support]" \
+                "($help)--shutdown-timeout=[Set the shutdown timeout value in seconds]:time: " \
                 "($help)*--storage-opt=[Storage driver options]:storage driver options: " \
                 "($help)--tls[Use TLS]" \
                 "($help)--tlscacert=[Trust certs signed only by this CA]:PEM file:_files -g \"*.(pem|crt)\"" \