[style]Fix style in user_datatable_spec

spec/datatables/user_datatable_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 
 describe UserDatatable do
-  subject! do
+  subject!(:user_datatable) do
     described_class.new(view)
   end
 
@@ -92,7 +92,7 @@ describe UserDatatable do
 
   skip 'outputs' do
     let(:user) { User.first }
-    let(:output) { subject.as_json }
+    let(:output) { user_datatable.as_json }
 
     it 'recordsTotal' do
       expect(output[:recordsTotal]).to eq(1)