README: fix typo in code example description

[irgolic]

cogames.policy.Policy => mettagrid.policy.policy.MultiAgentPolicy

[irgolic]

Looking deeper into the code example, it looks really out of date.

from mettagrid.policy.policy import MultiAgentPolicy as Policy

class MyPolicy(Policy):
    def __init__(self, observation_space, action_space):
        self.network = MyNetwork(observation_space, action_space)

The __init__ doesn't call the parent's __init__, which requires a PolicyEnvInterface argument.

self.network is a () -> None | nn.Module method in the parent, but overriden as an instance attribute in the subclass.

Beyond this example, and the scripted agents, I couldn't find any python policy examples.