update link to slack docs

Author: olegs-codefresh

stable/slack/README.md

@@ -14,5 +14,8 @@ envs:
   - name: WEBHOOK_URL
     type: required
     description: Url to the channel
+  - name: SLACK_MESSAGE
+    type: required
+    description: Message to send
   - name: DEBUG
     description: print verbose install output

stable/slack/plugin.yaml

@@ -22,5 +22,5 @@ steps:
 
 ## Environment Variables
 
-- **required** `WEBHOOK_URL` - Url to the channel. Slack official [docs](https://gophers.slack.com/apps/new/A0F7XDUAZ-incoming-webhooks)
+- **required** `WEBHOOK_URL` - Url to the channel. Slack official [docs](https://api.slack.com/incoming-webhooks)
 - `DEBUG` - print verbose output