"fix" Debian/Ubuntu instructions

I’d like to suggest that the contents of the sources.list.d file for seadrive be changed to:

deb [arch=amd64] http://linux-clients.seafile.com/seadrive-deb/focal/ stable main

because, currently, doing an apt update gives an error about not supporting i386 architecture (on Ubuntu 20.04).

Thanks we’ll do the fix.