fix broken CONTRIBUTING.md link

Author: JounQin

README.md

@@ -118,7 +118,7 @@ new PurgecssPlugin({
 
 ## Contributing
 
-Please read [CONTRIBUTING.md](./.github/CONTRIBUTING.md) for details on our code of conduct, and the process for submitting pull requests to us.
+Please read [CONTRIBUTING.md](./CONTRIBUTING.md) for details on our code of conduct, and the process for submitting pull requests to us.
 
 ## Versioning