Update slack.mdx

Fixed Issue: novuhq#238 changed ChatProviderIdEnum. to ChatProviderIdEnum.Slack

Author: devshah7774

channels-and-providers/chat/slack.mdx

@@ -70,7 +70,7 @@ Create a new endpoint on your server that will handle the following steps (you c
     const novu = new Novu("<NOVU_API_KEY>");
 
     const body = req.body; // From your HTTPS listener
-    await novu.subscribers.setCredentials('subscriberId', ChatProviderIdEnum., {
+    await novu.subscribers.setCredentials('subscriberId', ChatProviderIdEnum.Slack, {
     webhookUrl: body.incoming_webhook.url,
     });
     ```
@@ -115,4 +115,4 @@ In order to enable Hash-Based Message Authentication Codes, you need to do the f
 
 3. Add the newly created hash HMAC to the Sharable URL as a query.
 
-This concludes the Slack provider guide.
+This concludes the Slack provider guide.