fix: correct the validation message

subcommands/expose

@@ -13,7 +13,7 @@ service-expose-cmd() {
   local cmd="$PLUGIN_COMMAND_PREFIX:expose" argv=("$@"); [[ ${argv[0]} == "$cmd" ]] && shift 1
   declare SERVICE="$1" PORTS_LIST="${@:2}"
 
-  [[ -z "$SERVICE" ]] && dokku_log_fail "Please specify a name for the service"
+  [[ -z "$SERVICE" ]] && dokku_log_fail "Please specify a valid name for the service"
   verify_service_name "$SERVICE"
   service_port_expose "$SERVICE" "${@:2}"
 }