typo in learning lab FDM

[chrivand]

https://developer.cisco.com/learning/devnet-express/devnet-express-security-v1-1/dne-firepower-v11/fdm-restapi-handson-v-1-1/step/4

$ python3 fdm_create_networks.py

should be (otherwise file not found error):

$ python3 fdm_create_network.py