Update Slack app invitation command in documentation

The command to invite the app is `/invite` and it needs an @ to select the app

Author: francoiscampbell

pages/pipelines/integrations/notifications/slack_workspace.md

@@ -27,7 +27,7 @@ notify:
 
 ### Notify in private channels
 
-You can notify individuals in private channels by inviting the Buildkite Builds Slack App into the channel with `/add Buildkite Builds`.
+You can notify individuals in private channels by inviting the Buildkite Builds Slack App into the channel with `/invite @Buildkite Builds`.
 
 Build-level notifications: