fix documentation on decode_item

deprecated/removed `:decode_sets` option

Author: moperacz

lib/ex_aws/dynamo.ex

@@ -123,14 +123,6 @@ defmodule ExAws.Dynamo do
   |> ExAws.request!
   |> Dynamo.decode_item(as: User)
   ```
-
-  By default this will decode various dynamodb sets into lists. If you'd
-  prefer they be decoded into `MapSet`s instead, set `decode_sets: true` in
-  your compile-time (not runtime) configuration:
-
-  ```elixir
-  config :ex_aws, :dynamodb, decode_sets: true
-  ```
   """
   @spec decode_item(Map.t()) :: Map.t()
   @spec decode_item(Map.t(), as: atom) :: Map.t()