Fix example in Rails/PluralizationGrammar

Author: masato-bkn

lib/rubocop/cop/rails/pluralization_grammar.rb

@@ -11,7 +11,7 @@ module Rails
       #   3.day.ago
       #   1.months.ago
       #   5.megabyte
-      #   1.gigabyte
+      #   1.gigabytes
       #
       #   # good
       #   3.days.ago