Fix usage message of map_builder_server_main.cc (#1302)
Fix usage message of map_builder_server_main.cc (#1302)


cartographer-project/cartographer,cartographer-project/cartographer,googlecartographer/cartographer,damonkohler/cartographer,wohe/cartographer,googlecartographer/cartographer,googlecartographer/cartographer,cartographer-project/cartographer,wohe/cartographer,cartographer-project/cartographer,wohe/cartographer,wohe/cartographer,googlecartographer/cartographer,larics/cartographer,damonkohler/cartographer,larics/cartographer,larics/cartographer,damonkohler/cartographer,damonkohler/cartographer,larics/cartographer