Cookbook generics examples is incoherent

The code in [last section of the Generics part of the Cookbook](https://docs.servant.dev/en/stable/cookbook/generic/Generic.html#using-generics-together-with-a-custom-monad) is hard to grasp for a beginner.

Consider the following code from the Cookbook:

```
data AppCustomState =
AppCustomState

type AppM = ReaderT AppCustomState Handler

apiMyMonad :: Proxy (ToServantApi Routes)
apiMyMonad = genericApi (Proxy :: Proxy Routes)

getRouteMyMonad :: Int -> AppM String
getRouteMyMonad = return . show

putRouteMyMonad :: Int -> AppM Bool
putRouteMyMonad = return . odd

recordMyMonad :: Routes (AsServerT AppM)
recordMyMonad = Routes {_get = getRouteMyMonad, _put = putRouteMyMonad}

-- natural transformation
nt :: AppCustomState -> AppM a -> Handler a
nt s x = runReaderT x s

appMyMonad :: AppCustomState -> Application
appMyMonad state = genericServeT (nt state) recordMyMonad
```

However, nowhere is `apiMyMonad` used in this code.

I will happily take the task of submitting a patch, if this example can be rewritten to be right.

cc @arianvp