Adjust documentation

docs/notifications.md

@@ -124,6 +124,7 @@ To send notifications via shoutrrr, the following command-line options, or their
 - `--notification-url` (env. `WATCHTOWER_NOTIFICATION_URL`): The shoutrrr service URL to be used.
 
 Go to [https://github.com/containrrr/shoutrrr#service-urls](https://github.com/containrrr/shoutrrr#service-urls) to learn more about the different service URLs you can use.
+You can define multiple services by space separating the URLs. (See example below)
 
 Example:
 
@@ -132,7 +133,6 @@ docker run -d \
   --name watchtower \
   -v /var/run/docker.sock:/var/run/docker.sock \
   -e WATCHTOWER_NOTIFICATIONS=shoutrrr \
-  -e WATCHTOWER_NOTIFICATION_URL=discord://channel/token \
-  -e WATCHTOWER_NOTIFICATION_URL=slack://watchtower@token-a/token-b/token-c \
+  -e WATCHTOWER_NOTIFICATION_URL="discord://channel/token slack://watchtower@token-a/token-b/token-c" \
   containrrr/watchtower
 ```