Update serve change listening port documentation (#6229)

* [docs] Change serve port flag and add example

* [docs] Add --listen flag

docusaurus/docs/deployment.md

@@ -15,7 +15,11 @@ npm install -g serve
 serve -s build
 ```
 
-The last command shown above will serve your static site on the port **5000**. Like many of [serve](https://github.com/zeit/serve)’s internal settings, the port can be adjusted using the `-p` or `--port` flags.
+The last command shown above will serve your static site on the port **5000**. Like many of [serve](https://github.com/zeit/serve)’s internal settings, the port can be adjusted using the `-l` or `--listen` flags:
+
+```sh
+serve -s build -l 4000
+```
 
 Run this command to get a full list of the options available: