associations.cheatmd: Fix many_to_many example schemas

Author: xtian

guides/cheatsheets/associations.cheatmd

@@ -124,13 +124,11 @@ end
 #### The first schema
 
 ```elixir
-defmodule Movie do
+defmodule User do
   use Ecto.Schema
 
-  schema "movies" do
-    field :title, :string
-    field :release_date, :date
-    many_to_many :actors, Actor, join_through: "movies_actors"
+  schema "users" do
+    many_to_many :organizations, Organization, join_through: "users_organizations"
   end
 end
 ```
@@ -139,12 +137,11 @@ end
 #### The second schema
 
 ```elixir
-defmodule Actor do
+defmodule Organization do
   use Ecto.Schema
 
-  schema "actors" do
-    field :name, :string
-    many_to_many :movies, Movie, join_through: "movies_actors"
+  schema "organizations" do
+    many_to_many :users, Organization, join_through: "users_organizations"
   end
 end
 ```