fix readme

Author: jojozhuang

README.md

@@ -39,7 +39,7 @@ Follow tutorial [Deploying Full Stack React App to Heroku](https://jojozhuang.gi
 
 Follow tutorial [Continuously Deploy Full Stack React App to Heroku and Netlify with Travis-CI](https://jojozhuang.github.io/tutorial/continuously-deploy-full-stack-react-app-to-heroku-and-netlify-with-travis-ci) to continuously deploy this Full Stack app to Heroku(RESTful API) and Netlify(Frontend React).
 
-## Upgrade eo heorku20
+## Upgrade to heorku20 stack
 Server.
 ```sh
 cd code-editor-react
@@ -57,6 +57,7 @@ heroku git:remote -a code-editor-react
 git commit --allow-empty -m "Upgrading to heroku-20"
 git push heroku master
 ```
+* [Upgrading to the Latest Stack](https://devcenter.heroku.com/articles/upgrading-to-the-latest-stack)
 
 # Portfolio
 Read portfolio [Code Editor(React)](https://jojozhuang.github.io/project/code-editor-react/) to learn the main functions of this code editor.