Fixed the link url to the list of middlewares on docs/api/index.md.

docs/api/index.md

@@ -126,7 +126,7 @@ http.createServer(app.callback()).listen(3001);
 
 ## app.use(function)
 
-  Add the given middleware function to this application. See [Middleware](#middleware) for
+  Add the given middleware function to this application. See [Middleware](https://github.com/koajs/koa/wiki#middleware) for
   more information.
 
 ## app.keys=