[docs] Stale ModelSpec / parallelize.py names after #4810

[YeonwooSung]

#4810 deleted ModelSpec and per-model parallelize.py. Trainer.Config.model is the Suppress-annotated field. Several docs still use the old names, so copy-paste snippets fail with TypeError: unexpected keyword argument 'model_spec' (or point at a file that no longer exists).

Stale references

• torchtitan/config/README.md: Trainer.Config(model_spec=...) and the CLI-freeze paragraph still says Trainer.Config.model_spec
• torchtitan/rl/README.md: Controller.Config(model_spec=...)
• torchtitan/overrides/README.md: example FQN model_spec.model.layers.0.feed_forward
• torchtitan/models/llama3/README.md: "Coverage below matches parallelize.py"

Local variables named model_spec = model_registry(...) in float8/mxfp8 docs are fine; those are not the config field.

Related: #4810, #4805

[tianyu-l]

thanks, please help update