Fix README example schema (#207)

Since Ecto 2.0 the model module doesn't exist anymore. Instead Ecto.Schema is used

Author: leifg

README.md

@@ -82,7 +82,7 @@ defmodule Repo do
 end
 
 defmodule Weather do
-  use Ecto.Model
+  use Ecto.Schema
 
   schema "weather" do
     field :city     # Defaults to type :string