A few code corrections

Writing-Middleware.md

@@ -14,15 +14,15 @@ Handlers are usually a struct with at least one field, the next Handler in the c
 
 ```go
 type MyHandler struct {
-	Next middleware.HandlerFunc
+	Next middleware.Handler
 }
 ```
 
-It must implement the middleware.Handler interface. We do this by adding a method called ServeHTTP, which happens to be a middlware.HandlerFunc. Right now, let's just pass the request to the next Handler in the chain:
+It must also implement the middleware.Handler interface. We do this by adding a method called ServeHTTP. Right now, let's just pass the request to the next Handler in the chain:
 
 ```go
 func (h MyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
-	return h.Next(w, r)
+	return h.Next.ServeHTTP(w, r)
 }
 ```