Update docs for Task.async_stream, closes #6395

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/elixir/lib/task.ex

@@ -345,9 +345,9 @@ defmodule Task do
   terminates all tasks.
 
   When streamed, each task will emit `{:ok, value}` upon successful
-  completion or `{:exit, reason, element}` if the caller is trapping
-  exits, where `element` is the stream element. Results are emitted
-  in the same order as the original `enumerable`.
+  completion or `{:exit, reason}` if the caller is trapping exits,
+  where `element` is the stream element. Results are emitted in the
+  same order as the original `enumerable`.
 
   The level of concurrency can be controlled via the `:max_concurrency`
   option and defaults to `System.schedulers_online/0`. A timeout