nixos/sane: bump the MaxConnections to a reasonable amount

Whenever I try to scan from another computer it has to establish >2
connections in order to succeed. With the connections being limited to 1
I can not scan any document.

This is also what other distributions ([Debian], [ArchLinux], …) have
done in one way or another.

[Debian]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=850649#5
[ArchLinux]: no limit: https://github.com/archlinux/svntogit-packages/blob/99cba454bb0b69034bc45e97cde4a460bccfef4b/trunk/saned.socket#L4

nixos/modules/services/hardware/sane.nix

@@ -148,7 +148,7 @@ in
           # saned needs to distinguish between IPv4 and IPv6 to open matching data sockets.
           BindIPv6Only = "ipv6-only";
           Accept = true;
-          MaxConnections = 1;
+          MaxConnections = 64;
         };
       };