fix documentation of REXML::Formatters::Default (id_hack -> ie_hack) [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59591 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rexml/formatters/default.rb

@@ -2,7 +2,7 @@
 module REXML
   module Formatters
     class Default
-      # Prints out the XML document with no formatting -- except if id_hack is
+      # Prints out the XML document with no formatting -- except if ie_hack is
       # set.
       #
       # ie_hack::