Update ddp_tutorial.rst (#2516)

1. add `dist.destroy_process_group()` in example code block
2. modify the link syntax error about torchrun

intermediate_source/ddp_tutorial.rst

@@ -340,11 +340,12 @@ Let's still use the Toymodel example and create a file named ``elastic_ddp.py``.
         labels = torch.randn(20, 5).to(device_id)
         loss_fn(outputs, labels).backward()
         optimizer.step()
+        dist.destroy_process_group()
         
     if __name__ == "__main__":
         demo_basic()
 
-One can then run a `torch elastic/torchrun<https://pytorch.org/docs/stable/elastic/quickstart.html>`__ command 
+One can then run a `torch elastic/torchrun <https://pytorch.org/docs/stable/elastic/quickstart.html>`__ command 
 on all nodes to initialize the DDP job created above:
 
 .. code:: bash