Update screenshot on app store

[marcelklehr]

The screenshot is outdated nowadays.

[joshtrichards]

Only 6 years old. :-P

nextcloud/screenshots#45

If someone wants to submit a PR similar to the above with an updated screenshot, have at it.

Maybe add more than one if feeling fancy.

References and examples:

• https://nextcloudappstore.readthedocs.io/en/latest/developer.html#info-xml
• https://github.com/nextcloud/deck/blob/0387895217467c973f566cb06aaad5e0a65d8ece/appinfo/info.xml#L39-L40
• nextcloud/mail#8838