lib/irb/locale.rb: fix typo

* lib/irb/locale.rb (IRB::Locale#modifier): fix wrong attr_reader
  `modifieer` => `modifier` from irb locale.  [fix GH-656]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46700 b2dd03c8-39d4-4d8f-98ff-823fe69b080e