Update associations.cheatmd

Author: xtian

guides/cheatsheets/associations.cheatmd

@@ -141,7 +141,7 @@ defmodule Organization do
   use Ecto.Schema
 
   schema "organizations" do
-    many_to_many :users, Organization, join_through: "users_organizations"
+    many_to_many :users, User, join_through: "users_organizations"
   end
 end
 ```