[ruby/error_highlight] Fixed the argument for DidYouMean.formatter=

Looks like this bug was hidden by did_you_mean's rescuing any
exceptions.

https://github.com/ruby/error_highlight/commit/7a8f0b4796

test/error_highlight/test_error_highlight.rb

@@ -13,7 +13,7 @@ class ErrorHighlightTest < Test::Unit::TestCase
   def setup
     if defined?(DidYouMean)
       @did_you_mean_old_formatter = DidYouMean.formatter
-      DidYouMean.formatter = DummyFormatter
+      DidYouMean.formatter = DummyFormatter.new
     end
   end