Fixed links in process_group_cpp_extension_tutorial.rst #3424

Job	Run time
pytorch_tutorial_build_worker (1, 15, linux.g5.12xlarge.nvidia.gpu)	12m 40s
pytorch_tutorial_build_worker (2, 15, linux.g5.4xlarge.nvidia.gpu)	51m 6s
pytorch_tutorial_build_worker (3, 15, linux.g5.4xlarge.nvidia.gpu)	19m 6s
pytorch_tutorial_build_worker (4, 15, linux.g5.4xlarge.nvidia.gpu)	30m 50s
pytorch_tutorial_build_worker (5, 15, linux.g5.4xlarge.nvidia.gpu)	20m 7s
pytorch_tutorial_build_worker (6, 15, linux.g5.4xlarge.nvidia.gpu)	17m 31s
pytorch_tutorial_build_worker (7, 15, linux.4xlarge.nvidia.gpu)	19m 16s
pytorch_tutorial_build_worker (8, 15, linux.4xlarge.nvidia.gpu)	11m 47s
pytorch_tutorial_build_worker (9, 15, linux.4xlarge.nvidia.gpu)	19m 36s
pytorch_tutorial_build_worker (10, 15, linux.4xlarge.nvidia.gpu)	16m 2s
pytorch_tutorial_build_worker (11, 15, linux.4xlarge.nvidia.gpu)	14m 48s
pytorch_tutorial_build_worker (12, 15, linux.4xlarge.nvidia.gpu)	29m 50s
pytorch_tutorial_build_worker (13, 15, linux.4xlarge.nvidia.gpu)	41m 54s
pytorch_tutorial_build_worker (14, 15, linux.4xlarge.nvidia.gpu)	36m 1s
pytorch_tutorial_build_worker (15, 15, linux.4xlarge.nvidia.gpu)	25m 24s
pytorch_tutorial_build_manager	18m 48s
	6h 24m 46s

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