[SYCL][Doc] Fix root-group example

[gmlueck]

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.