Update onListening example in readme

[victoria-miltcheva]

I tried using the code in the onListening example, but was getting this error:

TypeError: server.getAddress is not a function
    at Object.onListening (<redacted>/rollup.config.js:45:32)
    at Server.<anonymous> (<redacted>/node_modules/rollup-plugin-serve/dist/index.cjs.js:85:74)
    at Object.onceWrapper (events.js:286:20)

After some troubleshooting, I noticed that server.getAddress() has been renamed to server.address(). Additionally, the host property of the returned object has been renamed to address. Making these changes allowed me to run the code successfully.

I wanted to update it here as well to save people some troubleshooting time 🙂.