Fix plugin update docs: use /plugin update command

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mnvsk97

docs-site/docs/getting-started/plugin.md

@@ -21,7 +21,7 @@ pip install agentbreak
 To update after a new release:
 
 ```
-/plugin marketplace add mnvsk97/agentbreak
+/plugin update agentbreak@mnvsk97-agentbreak
 /reload-plugins
 ```