Fix indent (#843)

## Description
Indent of `end` is different from that of accompanying `it` clause.
So I fixed it.

spec/draper/decoratable_spec.rb

@@ -11,7 +11,7 @@ module Draper
 
         expect(decorator).to be_a ProductDecorator
         expect(decorator.object).to be product
-     end
+      end
 
       it "accepts context" do
         context = {some: "context"}