Fixup app.use links in 4x docs

 [req.originalUrl](http://expressjs.com/api.html#req.originalUrl) and [router.user](http://expressjs.com/api.html#router.use) sections contain badly formatted links to `app.use`.
Merge pull request #264 from jgable/fixAppUseLink

Fixup app.use links in 4x docs