Fix the PPO example docstrings to name the arguments the constructors take

[darkdi]

What does this PR do?

The two policy-network constructors in the PPO domain template document arguments they do not take. Both say:

Args:
    input_shape: observation shape of the environment
    n_actions: number of discrete actions available in the environment

but ActorCategorical.__init__ takes only actor_net, and ActorContinuous.__init__ takes actor_net and act_dim. The entries are left over from a version that built the network inside the constructor — ExperienceSourceDataset above them still takes the network from the caller.

This is a template people copy, so the wrong names travel. Replaced the four entries with the arguments that are actually there. Docstrings only; nothing else in the file changes.

One related thing I left alone: LambdaCallback documents **kwargs, while its __init__ spells out every hook explicitly and has no **kwargs. Strictly it is the same defect, but the honest fix would mean listing fifty keyword arguments or rewording the entry, and that is your call rather than mine.

Before submitting

• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes? — no behaviour change; ruff check and ruff format --check pass on the file with the pinned ruff==0.15.9
• Did you list all the breaking changes introduced by this pull request? — none
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.