[SYCL][Doc] Fix root-group example (#20817)
Fix several problems with the example in this extension:
* The `use_root_sync` property is a kernel property, not a kernel launch
property. Therefore, it should decorate the kernel definition, not be
passed to the launch command.
* The `max_num_work_group_sync` was renamed to `max_num_work_groups` a
long time ago, but it seems like we forgot to update this example.
* Upgrade the example from a code snippet to a full example. I verified
that it compiles and runs correctly.
