test: use double quote for string literal

Author: kou

test/parse/test_processing_instruction.rb

@@ -82,7 +82,7 @@ def test_after_root
     def test_linear_performance_gt
       seq = [10000, 50000, 100000, 150000, 200000]
       assert_linear_performance(seq, rehearsal: 10) do |n|
-        REXML::Document.new('<?xml version="1.0" ' + ">" * n + ' ?>')
+        REXML::Document.new("<?xml version=\"1.0\" " + ">" * n + " ?>")
       end
     end
   end