Clarify the message name, which is 'm'. (#4462)

Co-authored-by: Ubuntu <ubuntu@ip-172-31-34-29.ap-northeast-1.compute.internal>
Co-authored-by: Rhett Ying <85214957+Rhett-Ying@users.noreply.github.com>

tutorials/blitz/3_message_passing.py

@@ -123,8 +123,8 @@ class SAGEConv(nn.Module):
 # three concepts here:
 #
 # * Message function ``fn.copy_u('h', 'm')`` that
-#   copies the node feature under name ``'h'`` as *messages* sent to
-#   neighbors.
+#   copies the node feature under name ``'h'`` as *messages* with name
+#   ``'m'`` sent to neighbors.
 #
 # * Reduce function ``fn.mean('m', 'h_N')`` that averages
 #   all the received messages under name ``'m'`` and saves the result as a