fix comma placement typo

lib/koans/16_tasks.ex

@@ -41,7 +41,7 @@ defmodule Tasks do
   end
 
   koan "You can yield to multiple tasks at once and extract the results" do
-    squares = [1 ,2 ,3 ,4]
+    squares = [1, 2, 3, 4]
              |> Enum.map(fn(number) -> Task.async(fn -> number * number end) end)
              |> Task.yield_many(100)
              |> Enum.map(fn({_task,{:ok, result}}) -> result end)