core: add missing "=" in message

For consistency. Also drop "e.g." because it's somewhat redundant with the
ellipsis and the message is pretty long already.

Follow-up for 4d1fe20a58.

src/core/socket.c

@@ -449,7 +449,7 @@ static int socket_verify(Socket *s) {
                 return 0;
 
         if (!s->ports) {
-                log_unit_error(UNIT(s), "Unit has no Listen setting (e.g. ListenStream=, ListenDatagram=, ListenFIFO, ...). Refusing.");
+                log_unit_error(UNIT(s), "Unit has no Listen setting (ListenStream=, ListenDatagram=, ListenFIFO=, ...). Refusing.");
                 return -EINVAL;
         }