minor, fix dev_env.md about how to download JS
minor, fix dev_env.md about how to download JS

haoch/kylin,murkrishn/incubator-kylin,haoch/kylin,haoch/kylin,murkrishn/incubator-kylin,lemire/incubator-kylin,murkrishn/incubator-kylin,lemire/incubator-kylin,lemire/incubator-kylin,haoch/kylin,lemire/incubator-kylin,haoch/kylin,murkrishn/incubator-kylin,haoch/kylin