docs: Fix outdated slack notification configuration readme (#305)

Signed-off-by: Yating Zhou <[email protected]>

docs/services/slack.md

@@ -21,11 +21,11 @@ The Slack notification service configuration includes following settings:
 
 1. Create Slack Application using https://api.slack.com/apps?new_app=1
 ![1](https://user-images.githubusercontent.com/426437/73604308-4cb0c500-4543-11ea-9092-6ca6bae21cbb.png)
-1. Once application is created navigate to `Enter OAuth & Permissions`
+1. Once application is created navigate to `OAuth & Permissions`
 ![2](https://user-images.githubusercontent.com/426437/73604309-4d495b80-4543-11ea-9908-4dea403d3399.png)
-1. Click `Permissions` under `Add features and functionality` section and add `chat:write` scope. To use the optional username and icon overrides in the Slack notification service also add the `chat:write.customize` scope.
+1. Go to `Scopes` > `Bot Token Scopes` > `Add an OAuth Scope`. Add `chat:write` scope. To use the optional username and icon overrides in the Slack notification service also add the `chat:write.customize` scope.
 ![3](https://user-images.githubusercontent.com/426437/73604310-4d495b80-4543-11ea-8576-09cd91aea0e5.png)
-1. Scroll back to the top, click 'Install App to Workspace' button and confirm the installation.
+1. `OAuth & Permission` > `OAuth Tokens for Your Workspace` > `Install to Workspace`
 ![4](https://user-images.githubusercontent.com/426437/73604311-4d495b80-4543-11ea-9155-9d216b20ec86.png)
 1. Once installation is completed copy the OAuth token.
 ![5](https://user-images.githubusercontent.com/426437/73604312-4d495b80-4543-11ea-832b-a9d9d5e4bc29.png)