Documentation: Provide exact location where an XOS service is created

In Example service documentation (https://guide.opencord.org/xos/example_service.html), under "Define a Model" section, we specify that we should "create a file named exampleservice.xproto in your service's xos directory". There is however no context/information about where the XOS directory lives.

We need to update the documentation to specify where exactly in the Cord directory tree we want to create the example service xproto. Otherwise a newbie would not know how to proceed.