Add cluster in collaborative-whiteboard.mdx (#1862)

Update collaborative-whiteboard.mdx

docs/javascript/v2/how-to-guides/enable-content-sharing/collaborative-whiteboard.mdx

@@ -49,6 +49,6 @@ APP_CLUSTER="cluster"
 ## Whiteboard Client
 
 -   From the [100ms-web sample app](https://github.com/100mslive/100ms-web) repository, copy the whole folder at `/src/plugins/whiteboard` into your live video conferencing or live streaming apps using 100ms' SDKs.
--   Add the Pusher `app key` and Pusher auth endpoint to `REACT_APP_PUSHER_APP_KEY` and `REACT_APP_PUSHER_AUTHENDPOINT` environment variables.
+- Add the pusher app key, auth endpoint and cluster to `REACT_APP_PUSHER_APP_KEY`, `REACT_APP_PUSHER_AUTHENDPOINT`, `REACT_APP_PUSHER_CLUSTER` environment variables.
 -   The `useWhiteboardMetadata` hook returns a state such as the whiteboard owner(`whiteboardOwner`) and action to toggle the whiteboard(`toggleWhiteboard`). Refer to usage in `ToggleWhiteboard.jsx` - an icon button to toggle the whiteboard based on the active state.
 -   When the whiteboard is active(`whiteboardOwner` from `useWhiteboardMetadata` is not null), render the `Whiteboard` component on your UI to let your users draw on the whiteboard. Refer `mainView.jsx` and `WhiteboardView.jsx`.