flipped Order[A @@ Dual]

Also includes a doc fix for Tags.Dual.

```scala
scala> :paste
// Entering paste mode (ctrl-D to finish)

import scalaz.{@@, Tag, Tags}
import scalaz.std.anyVal._
import scalaz.std.list._
import scalaz.syntax.foldable._
import scalaz.Dual._
val nums: List[Int @@ Tags.Dual] = Tag subst List(4,1,2,33,8)
(nums.minimum, (nums: List[Int]).minimum)

// Exiting paste mode, now interpreting.

nums: List[scalaz.package.@@[Int,scalaz.Tags.Dual]] = List(4, 1, 2, 33, 8)
res2: (Option[scalaz.package.@@[Int,scalaz.Tags.Dual]], Option[Int])
  = (Some(33),Some(1))
```