Outdated example code in `README.md` for running the device (local build)

[bj-s]

The example code for running the device after building ist not working:

karabo-cppserver serverId=cppServer/1 deviceClasses=AravisCameras

Starting a server as the example states, no device classes are available in Karabo for device creation. Instead, e.g. the following codes works. However this reduces the available device classes to only one of the installed classes:

karabo-cppserver serverId=cppServer/1 deviceClasses=AravisCamera