Use page permalink instead of link with path

lib/util/slack.js

@@ -160,7 +160,7 @@ module.exports = function(crowi) {
       author_link: url + '/user/' + user.username,
       author_icon: user.image,
       title: page.path,
-      title_link: url + page.path,
+      title_link: url + '/' + page._id,
       text: body,
       mrkdwn_in: ["text"],
     };