[Doc] fix simple typo, tranfer -> transfer (#2464)

There is a small typo in python/dgl/dataloading/async_transferer.py.

Should read `transfer` rather than `tranfer`.
Co-authored-by: Minjie Wang <wmjlyjemaine@gmail.com>

python/dgl/dataloading/async_transferer.py

@@ -13,7 +13,7 @@ class Transfer(object):
         Parameters
         ----------
         transfer_id : int
-            The id of the asynchronous tranfer.
+            The id of the asynchronous transfer.
         handle : DGLAsyncTransferer
             The handle of the DGLAsyncTransferer object that initiated the
             transfer.