Update mattermost-v11-changelog.md

Author: amyblais

source/product-overview/mattermost-v11-changelog.md

@@ -21,6 +21,7 @@ If you upgrade from a release earlier than v11.1, please read the other [Importa
 ### Improvements
 
 #### User Interface (UI)
+ - Pre-packaged Playbooks plugin [v2.6.0](https://github.com/mattermost/mattermost-plugin-playbooks/releases/tag/v2.6.0).
  - Pre-packaged Zoom plugin [v1.10.0](https://github.com/mattermost/mattermost-plugin-zoom/releases/tag/v1.10.0).
  - Pre-packaged Agents plugin [v1.6.0](https://github.com/mattermost/mattermost-plugin-agents/releases/tag/v1.6.0).
  - Pre-packaged ServiceNow plugin [v2.4.0](https://github.com/mattermost/mattermost-plugin-servicenow/releases/tag/v2.4.0).