use --format=text for discovery guide join token

docs/pages/auto-discovery/databases.mdx

@@ -33,7 +33,7 @@ Auth Service and save it in `/tmp/token` on the host that will run the
 Discovery Service.
 
 ```code
-$ tctl tokens add --type=discovery
+$ tctl tokens add --type=discovery --format=text
 ```
 
 (!docs/pages/includes/database-access/alternative-methods-join.mdx!)