error in GPU_MAPPING.md

[flashlau]

Error in file https://github.com/FedML-AI/FedML/blob/master/fedml_experiments/distributed/fedavg/GPU_MAPPING.md. In section "Skip some GPU devices inside a machine", configuration for client2 and client3 is wrong. It should be "client 2 -- host2:GPU:0, client 3 -- host2:GPU:2".

[chaoyanghe]

Thank you for your feedback. We will fix the wording.