fix "Nuisances" tutorial

tutorials/Nuisances.md

@@ -28,7 +28,7 @@ y = let x = 1 + 2 in x + x
 spidr will interpret each `x` as a different expression, and create two copies of `1 + 2`. This is acceptable for small calculations, but it would be a big problem if `x` were expensive to evaluate, or used a lot of space in memory. To prevent recalculating expressions, spidr provides _observable sharing_ via the interface
 > ```idris
 > interface Taggable a where
->   tag : a -> Tag a
+>   tag : Monad m => a -> TagT m a
 > ```
 `tag` tags all tensor expressions contained within the `a`. You can efficiently reuse a value created by `tag` as many times as you like; it will only be evaluated once. In our example, this would be
 ```idris