env.get_action_mask function accesses wrong member

[DocDriven]

I noticed that calling the 'get_action_mask' function on any environment fails with the error

>>> env.get_action_mask()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/conda/lib/python3.11/site-packages/nasim/envs/environment.py", line 427, in get_action_mask
    if self.network.host_discovered(action.target):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Network' object has no attribute 'host_discovered'

host_discovered is a method of the state class instead of the network class. It is a one-word fix.