man: use prefix number that matches the general suggestion

man: use prefix number that matches the general suggestion

`systemd.network(5)` recommends “that each filename is prefixed with a number
smaller than "70" (e.g.  10-eth0.network)”.

Reduce that used by the example accordingly, but stay above the number (`50`)
used in the earlier example for static configuration, so that would take
precedence over the dynamic one if both match for the same network.