Use the more idiomatic let in main_command_spec

instead of instance variable

spec/stackup/main_command_spec.rb

@@ -2,20 +2,21 @@
 
 describe Stackup::MainCommand do
   context "change-set create --service-role-arn"
+    let(:mock_change_set) { double() }
+
     before(:example) do
       mock_stackup = double()
       mock_stack = double()
-      @mock_change_set = double()
       allow_any_instance_of(Stackup::MainCommand).to receive(:Stackup).and_return(mock_stackup)
       allow(mock_stackup).to receive(:stack).and_return(mock_stack)
-      allow(mock_stack).to receive(:change_set).and_return(@mock_change_set)
+      allow(mock_stack).to receive(:change_set).and_return(mock_change_set)
     end
 
     it "invokes stack.change_set.create with role arn passed through" do
       expected_args = {
         role_arn: "arn:aws:iam::000000000000:role/example"
       }
-      expect(@mock_change_set).to receive(:create).with(hash_including(expected_args))
+      expect(mock_change_set).to receive(:create).with(hash_including(expected_args))
 
       Stackup::MainCommand.run("stackup", [
         "STACK-NAME", "change-set", "create",
@@ -28,7 +29,7 @@
         expected_args = {
           allow_empty_change_set: nil
         }
-        expect(@mock_change_set).to receive(:create).with(hash_including(expected_args))
+        expect(mock_change_set).to receive(:create).with(hash_including(expected_args))
 
         Stackup::MainCommand.run("stackup", [
           "STACK-NAME", "change-set", "create",
@@ -41,7 +42,7 @@
         expected_args = {
           allow_empty_change_set: true
         }
-        expect(@mock_change_set).to receive(:create).with(hash_including(expected_args))
+        expect(mock_change_set).to receive(:create).with(hash_including(expected_args))
 
         Stackup::MainCommand.run("stackup", [
           "STACK-NAME", "change-set", "create",