man: systemd-stdio-bridge doesn't connect "two busses"

In D-Bus, clients connect to a bus (the usual case), or use direct
questions to each other (the unusual case). A bus is a program one can
connect to and implemented by dbus-daemon or dbus-broker. HOwever,
busses never connect between each other, that doesn't exist. Hence don't
claim so.

This is probably confusion about the fact that sd-bus calls D-Bus
connection objects just "sd_bus" for simplicity, given they are used in
99% of the cases to connect to a bus — only in exceptional cases they
are used for direct connections between peers without involving a bus.

Follow-up for b7bb58ef70

src/stdio-bridge/stdio-bridge.c

@@ -27,7 +27,7 @@ static bool arg_user = false;
 
 static int help(void) {
         printf("%s [OPTIONS...]\n\n"
-               "Forward messages between two D-Bus busses via a pipe or socket.\n\n"
+               "Forward messages between a pipe or socket and a D-Bus bus.\n\n"
                "  -h --help              Show this help\n"
                "     --version           Show package version\n"
                "  -p --bus-path=PATH     Path to the bus address (default: %s)\n"