Fix admin port in shutdown example.
Fix admin port in shutdown example.

The code example in the "Stop the service" section currently lists port
9990 instead of the default admin port 9900. Changed the code example so
it's more straightforward for newcomers.

fizx/scala-bootstrapper,wuqiangxjtu/scala-bootstrapper,wuqiangxjtu/scala-bootstrapper,twitter/scala-bootstrapper,hydro2k/scala-bootstrapper,chintootech/scala-bootstrapper,chintootech/scala-bootstrapper,wuqiangxjtu/scala-bootstrapper,hydro2k/scala-bootstrapper,fizx/scala-bootstrapper,chintootech/scala-bootstrapper,twitter/scala-bootstrapper,hydro2k/scala-bootstrapper