Update rake example task to use ruby 2.5 parsing

Signed-off-by: Tim Smith <tsmith@chef.io>

tasks/cops_documentation.rake

@@ -91,7 +91,7 @@ begin
 
   desc 'Syntax check for the documentation comments'
   task documentation_syntax_check: :yard_for_generate_documentation do
-    require 'parser/ruby24'
+    require 'parser/ruby25'
 
     ok = true
     YARD::Registry.load!
@@ -106,7 +106,7 @@ begin
       examples.to_a.each do |example|
         buffer = Parser::Source::Buffer.new('<code>', 1)
         buffer.source = example.text
-        parser = Parser::Ruby24.new(RuboCop::AST::Builder.new)
+        parser = Parser::Ruby25.new(RuboCop::AST::Builder.new)
         parser.diagnostics.all_errors_are_fatal = true
         parser.parse(buffer)
       rescue Parser::SyntaxError => e