docs: fix env variable name `notification(s)-delay` (#1486)

docs/notifications.md

@@ -18,7 +18,7 @@ system, [logrus](http://github.com/sirupsen/logrus).
 
 -   `--notifications-level` (env. `WATCHTOWER_NOTIFICATIONS_LEVEL`): Controls the log level which is used for the notifications. If omitted, the default log level is `info`. Possible values are: `panic`, `fatal`, `error`, `warn`, `info`, `debug` or `trace`.
 -   `--notifications-hostname` (env. `WATCHTOWER_NOTIFICATIONS_HOSTNAME`): Custom hostname specified in subject/title. Useful to override the operating system hostname.
--   `--notifications-delay` (env. `WATCHTOWER_NOTIFICATION_DELAY`): Delay before sending notifications expressed in seconds.
+-   `--notifications-delay` (env. `WATCHTOWER_NOTIFICATIONS_DELAY`): Delay before sending notifications expressed in seconds.
 -   Watchtower will post a notification every time it is started. This behavior [can be changed](https://containrrr.github.io/watchtower/arguments/#without_sending_a_startup_message) with an argument.
 -   `notification-title-tag` (env. `WATCHTOWER_NOTIFICATION_TITLE_TAG`): Prefix to include in the title. Useful when running multiple watchtowers.
 -   `notification-skip-title` (env. `WATCHTOWER_NOTIFICATION_SKIP_TITLE`): Do not pass the title param to notifications. This will not pass a dynamic title override to notification services. If no title is configured for the service, it will remove the title all together.