FTP URL could be HTTPS URL on mobile-logos.html

[Andras Farkas <deepbluemistake%gmail.com@localhost>]

On
https://www.netbsd.org/gallery/mobile-logos.html
one link is an FTP URL:
ftp://ftp.netbsd.org/pub/NetBSD/misc/jschauma/background.jpg.bz2
Unfortunately, Mozilla has removed FTP support from Firefox, meaning
Firefox doesn't do anything useful upon following this link.
However, this URL works just fine:
https://ftp.netbsd.org/pub/NetBSD/misc/jschauma/background.jpg.bz2
An http: URL could also work just as well.

This is an optional change, but given most people will be looking at
mobile-logos.html in a web browser, it makes sense to use an HTTP or
HTTPS URL.