fix add-to-slack.mp4 link

slack.mdx

@@ -25,10 +25,10 @@ Once you’ve set up the integration in Plain, go to the Slack channels you want
 <video
   controls
   className="w-full aspect-video"
-  src="https://static-assets-git-add-slack-video-team-plain.vercel.app/docs/slack/add-to-slack.mp4"
+  src="https://static-assets.plain.com/docs/slack/add-to-slack.mp4"
 ></video>
 
-**3 Start responding **
+**3 Start responding 💬**
 
 Slack messages from the authorized channels will begin appearing as threads in Plain. To respond, press `R` and begin typing in Plain. Your responses will appear in Slack as if you are responding directly in the app.