Fix link to serveDirectory.

servant/src/Servant/API/Raw.hs

@@ -9,6 +9,6 @@ import           Data.Typeable (Typeable)
 -- a modified (stripped) 'pathInfo' if the 'Application' is being routed with 'Servant.API.Sub.:>'.
 --
 -- In addition to just letting you plug in your existing WAI 'Application's,
--- this can also be used with 'Servant.Utils.StaticFiles.serveDirectory' to serve
+-- this can also be used with <https://hackage.haskell.org/package/servant-server/docs/Servant-Utils-StaticFiles.html#v:serveDirectory serveDirectory> to serve
 -- static files stored in a particular directory on your filesystem
 data Raw deriving Typeable