Rename to correct name

Signed-off-by: Yuta Iwama <ganmacs@gmail.com>

lib/fluent/command/fluentd.rb

@@ -93,7 +93,7 @@
     begin
       opts[:log_rotate_age] = Integer(age)
     rescue TypeError, ArgumentError
-      usage "log-rotate-age should be #{rotate_ages.join(', ')} or a number"
+      usage "log-rotate-age should be #{ROTATE_AGE.join(', ')} or a number"
     end
   end
 }