help: Update broken link in `import from slack`.

Previously, the help center has a broken link that just points to
Slack's help center. With this change, we will remove and replace the
link to their OAuth scope section.

help/import-from-slack.md

@@ -56,7 +56,7 @@ in order to export direct message data.
    scratch" creation option.
 
 4. [Create a
-   bot user](https://slack.com/help/articles/115005265703-Create-a-bot-for-your-workspace),
+   bot user](https://api.slack.com/authentication/basics#scopes),
    following the instructions to add the following OAuth scopes to your bot:
     - `emoji:read`
     - `users:read`