fix: updated contributing markdown

Author: parthivrh

packages/react-core/CONTRIBUTING.md

@@ -79,17 +79,15 @@ $ yarn test
 
 Ensure no lint errors are introduced in `yarn-error.log` after running this command.
 
-5.  Add a commit using `yarn commit`:
+5.  Add a commit using `git commit`:
 
-This project uses [`lerna`](https://lernajs.io/) to do automatic releases and generate a changelog based on the commit history. So we follow [a convention][3] for commit messages. Please follow this convention for your commit messages.
-
-You can use `commitizen` to help you to follow [the convention][3].
+This project uses [`lerna`](https://lerna.js.io/) to do automatic releases and generate a changelog based on the commit history. So we follow [a convention][3] for commit messages. Please follow [`this convention`](https://github.com/angular/angular.js/blob/master/DEVELOPERS.md#type) for your commit messages.
 
 Once you are ready to commit the changes, please use the below commands:
 
 ```text
 $ git add <files to be committed>
-$ yarn commit
+$ git commit -m
 ```
 
 ... and follow the instruction of the interactive prompt.