Update model example to match Relish (#1919)

Author: joeltaylor

README.md

@@ -147,12 +147,14 @@ For example:
 ```ruby
 require "rails_helper"
 
-RSpec.describe User, :type => :model do
-  it "orders by last name" do
-    lindeman = User.create!(first_name: "Andy", last_name: "Lindeman")
-    chelimsky = User.create!(first_name: "David", last_name: "Chelimsky")
-
-    expect(User.ordered_by_last_name).to eq([chelimsky, lindeman])
+RSpec.describe Post, :type => :model do
+  context "with 2 or more comments" do
+    it "orders them in reverse chronologically" do
+      post = Post.create!
+      comment1 = post.comments.create!(:body => "first comment")
+      comment2 = post.comments.create!(:body => "second comment")
+      expect(post.reload.comments).to eq([comment2, comment1])
+    end
   end
 end
 ```