Update design-principles.md (#3314)

Updated link to React Devtools package repository

content/docs/design-principles.md

@@ -92,7 +92,7 @@ There is an internal joke in the team that React should have been called "Schedu
 
 Providing a good developer experience is important to us.
 
-For example, we maintain [React DevTools](https://github.com/facebook/react-devtools) which let you inspect the React component tree in Chrome and Firefox. We have heard that it brings a big productivity boost both to the Facebook engineers and to the community.
+For example, we maintain [React DevTools](https://github.com/facebook/react/tree/master/packages/react-devtools) which let you inspect the React component tree in Chrome and Firefox. We have heard that it brings a big productivity boost both to the Facebook engineers and to the community.
 
 We also try to go an extra mile to provide helpful developer warnings. For example, React warns you in development if you nest tags in a way that the browser doesn't understand, or if you make a common typo in the API. Developer warnings and the related checks are the main reason why the development version of React is slower than the production version.