Update app store screenshot(s)

[joshtrichards]

Same screenshot is used for other FTS apps and is from v0.3.0 (!) aka "Full Next Search".

https://github.com/nextcloud/fulltextsearch/tree/master/screenshots

https://github.com/nextcloud/files_fulltextsearch/blob/56c3d0caeb32c407425c6a0d86a5b9ded9cac2d4/appinfo/info.xml#L31

fulltextsearch/appinfo/info.xml

Line 28 in 21c5525

<screenshot>https://raw.githubusercontent.com/nextcloud/fulltextsearch/master/screenshots/0.3.0.png</screenshot>