fix documentation typo (#604)

Prior to this commit, the explanatory text about the async middleware
mispelled `reason` as `reasoon`.

lib/absinthe/middleware/async.ex

@@ -68,7 +68,7 @@ defmodule Absinthe.Middleware.Async do
  # we handle the different tuple results.
  #
  # The `put_result` function handles setting the appropriate state.
- # If the result is an `{:ok, value} | {:error, reasoon}` tuple it will set
+ # If the result is an `{:ok, value} | {:error, reason}` tuple it will set
  # the state to `:resolved`, and if it is another middleware tuple it will
  # set the state to unresolved.
  def call(%{state: :suspended} = res, {task, opts}) do