docs: update contributing path in README (#544)

Author: callmehiphop

handwritten/firestore/README.md

@@ -91,7 +91,7 @@ More Information: [Google Cloud Platform Launch Stages][launch_stages]
 
 ## Contributing
 
-Contributions welcome! See the [Contributing Guide](https://github.com/googleapis/nodejs-firestore/blob/master/.github/CONTRIBUTING.md).
+Contributions welcome! See the [Contributing Guide](https://github.com/googleapis/nodejs-firestore/blob/master/CONTRIBUTING.md).
 
 ## License