Typo in tutorial, dist_barrier should be dist.barrier (#2281)

Co-authored-by: Svetlana Karslioglu <svekars@fb.com>

Author: daleevans

intermediate_source/FSDP_tutorial.rst

@@ -249,7 +249,7 @@ We add the following code snippets to a python script “FSDP_mnist.py”.
 
         if args.save_model:
             # use a barrier to make sure training is done on all ranks
-            dist_barrier()
+            dist.barrier()
             # state_dict for FSDP model is only available on Nightlies for now
             states = model.state_dict()
             if rank == 0: