Update screenshots about the built-in updater

Steps to reproduce

1. Go to https://docs.nextcloud.com/server/15/admin_manual/maintenance/update.html
2. Attempt to match this to an installation of NC14 (upgrading to NC15)

Expected behaviour

You should get the notification on your screen like the doc says OR the doc should not say it.

Also your screen should look like the screenshot (of version 10) OR the screenshot should be of a supported version.

Actual behaviour

Tell us what happens instead

The notification to upgrade does not appear, and you can not find the upgrade buttons manually using the screenshot as a guide.

Server configuration

Operating system:
Ubuntu 16.04
Web server:
Apache2
Database:
mariaDB (redis caching)
PHP version:
7.0
Nextcloud version: (see Nextcloud admin page)
14
Updated from an older Nextcloud/ownCloud or fresh install:
Fresh install.
Where did you install Nextcloud from:
tar.gz

I think the other info is not necessary becuase this is a bug in the doc not the code.

Start with the maintenance update page at docs.nextcloud.com/server/15/admin_manual/maintenance/update.html and compare its updater instructions and screenshots with the supported Nextcloud version. Update the wording and screenshots so they match the current built-in updater, or remove claims that no longer apply. Done means the page no longer directs readers to outdated controls or behavior.