Fix simplifications script's help message (LAB-990)

[mfitz]

See https://arupdigital.atlassian.net/browse/LAB-990

After this change:

$ python scripts/simplify_network.py --help
usage: simplify_network.py [-h] -n NETWORK [-s SCHEDULE] -p PROJECTION
                           [-np PROCESSES] -od OUTPUT_DIR

Simplify a MATSim network by removing intermediate links from paths

optional arguments:
  -h, --help            show this help message and exit
  -n NETWORK, --network NETWORK
                        Location of the network.xml file
  -s SCHEDULE, --schedule SCHEDULE
                        Location of the schedule.xml file
  -p PROJECTION, --projection PROJECTION
                        The projection network is in, eg. "epsg:27700"
  -np PROCESSES, --processes PROCESSES
                        The number of processes to split computation across
  -od OUTPUT_DIR, --output_dir OUTPUT_DIR
                        Output directory for the simplified network