Update docs slack->chat

Change-Id: I5cc399385371bb6993a001aa444adf29fd51ed56
Signed-off-by: Ry Jones <rjones@linuxfoundation.org>

docs/CONTRIBUTING.md

@@ -9,7 +9,7 @@ Before taking actions, we highly recommend reading the [docs](../README.md).
 We now have two channels for bug and questions:
 
 * [Jira](https://jira.hyperledger.org/secure/RapidBoard.jspa?rapidView=111): report bug issues, create to-do tasks.
-* [Slack](https://hyperledgerproject.slack.com/archives/cello): technical discussions and questions.
+* [Chat](https://chat.hyperledger.org/channel/cello): technical discussions and questions.
 
 Jira tasks with `To Do` status are available for picking. If you want to handle one, assign it to yourself, and update the status to `In Progress`. Remember to mark it to `Done` when the patch is merged.
 
@@ -80,4 +80,4 @@ $ git review
 
 After the review is uploaded successfully, you can open [Gerrit Dashboard](https://gerrit.hyperledger.org/r/#/dashboard/self) to invite reviewers for checking. The patch will be merged into the `master` branch if passing the reviewer checking.
 
-* If you need to refine the patch further, you can commit the new code with `git commit -a --amend`, and then repeat the `git review` command.
+* If you need to refine the patch further, you can commit the new code with `git commit -a --amend`, and then repeat the `git review` command.