Fix small missing equal sign for 'docker daemon --cluster-store-opt'

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

contrib/completion/zsh/_docker

@@ -586,7 +586,7 @@ __docker_subcommand() {
                 "($help)--default-gateway-v6[Container default gateway IPv6 address]:IPv6 address: " \
                 "($help)--cluster-store=[URL of the distributed storage backend]:Cluster Store:->cluster-store" \
                 "($help)--cluster-advertise=[Address of the daemon instance to advertise]:Instance to advertise (host\:port): " \
-                "($help)*--cluster-store-opt[Set cluster options]:Cluster options:->cluster-store-options" \
+                "($help)*--cluster-store-opt=[Set cluster options]:Cluster options:->cluster-store-options" \
                 "($help)*--dns=[DNS server to use]:DNS: " \
                 "($help)*--dns-search=[DNS search domains to use]:DNS search: " \
                 "($help)*--dns-opt=[DNS options to use]:DNS option: " \