docs(cloud_firestore): fix example dir path in CONTRIBUTING.md (#2282)

- packages/cloud_firestore/example is not exists. change to ./packages/cloud_firestore/cloud_firestore/example

Author: kichiemon

CONTRIBUTING.md

@@ -35,7 +35,7 @@ example's directory, run `pub get` to make sure its dependencies have been
 downloaded, and use `flutter run`. Make sure you have a device connected over
 USB and debugging enabled on that device.
 
- * `cd packages/cloud_firestore/example`
+ * `cd packages/cloud_firestore/cloud_firestore/example`
  * `flutter run`
 
 ## Running the tests